Guidelines for Treatment of Lateral Patella Dislocations in Skeletally Mature Patients

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Guidelines for Treatment of Lateral Patella Dislocations in Skeletally Mature Patients
Author(s)
Michael C. Liebensteiner, MD, PhD
Florian Dirisamer, MD
Peter Balcarek, MD
Philip Schoettle, MD, PhD

Take-Home Points

  • Lateral patella dislocation is sufficiently treated with modern versions of patellofemoral surgery.
  • Comprehensive assessment for underlying osseous pathology is paramount (torsional abnormalities of the femur or tibia, trochlea dysplasia, patella alta, etc).
  • In such cases, isolated medial patellofemoral ligament reconstructions will fail. Instead, the underlying osseous abnormalities must be addressed during concomitant procedures (derotational osteotomy, tibial tubercle transfer, trochleoplasty, etc).

The incidence of patellar instability is high, particularly in young females. In principle, cases of patellar instability can be classified as traumatic (dislocation is caused by external, often direct forces) or nontraumatic (anatomy predisposes to instability).1-4

Figure 1.
Because the vast majority of unstable patellae are unstable toward lateral and because instability is objective when the patella is fully dislocated, we use the term lateral patella dislocation (LPD) and refer to primary and recurrent LPD throughout this review.

Anatomy Predisposing to Patella Dislocation

Most patients present with specific anatomical factors that predispose to patellar instability (isolated or combined).

Figure 2.
These can be grossly categorized as osteochondral factors and soft-tissue factors.

Of the osteochondral factors, dysplasia of the femoral trochlea (trochlea groove [TG]) is most important. In healthy patients, the concave trochlea stabilizes the patella in knee flexion angles above 20°. In particular, the lateral facet of the trochlea plays a key role in withstanding the lateralizing quadriceps vector. The dysplastic trochlea, which has a flat or even a convex surface, destabilizes the patella (Figure 1). Moreover, patella alta is a pivotal factor in the development of LPD.

Figure 3.
A high-riding patella engages the femoral trochlea during higher degrees of knee flexion, making the patella very susceptible to dislocations when the knee is almost in extension.5,6 In addition, high femoral anteversion (increased femoral internal torsion) has been reported as contributing to the development of LPD. Internal torsion of the distal femur brings the TG more medial and therefore provokes a lateral shift of the patella relative to the femur (Figure 2).7-11 Valgus knee alignment is also common in patients with LPD.
Figure 4.
First, tibiofemoral valgus brings the tibial tuberosity (TT) more toward lateral and therefore increases the pull on the patella toward lateral. Second, when the deformity is at the distal femur, there is often a hypoplastic lateral condyle, which can contribute to LPD in knee flexion angles above 45°. Deformities in the frontal plane (valgus) and the transverse plane (increased internal torsion of the femur, increased external torsion of the proximal tibia) commonly increase the TT-TG distance. TT-TG distance is a radiographic parameter, taken from magnetic resonance imaging (MRI) or computed tomography, that summarizes important aspects of patellofemoral alignment and gives an impression of the amount of lateralizing force of the extensor apparatus (discussed later) (Figure 3).

The anteromedial soft tissue of the knee (retinaculum) has 3 layers, the second of which contains the
Figure 5.
medial patellofemoral ligament (MPFL).12,13 On the femoral side, the MPFL originates in direct proximity to the medial epicondyle and the adductor tubercle. The MPFL broadens toward the patella (V-shaped) and inserts at the superomedial border of the patella and the adjacent aspects of the quadriceps tendon.14-17
Figure 6.
It has been found to provide an important restraint against LPD.18-20 In primary LPD, the MPFL has been found ruptured or severely damaged in more than 90% of cases, most often near the femoral insertion.18,21-23 In patients with an elongated, insufficient MPFL, the patella may dislocate laterally without rupturing the MPFL. Another soft-tissue structure that contributes to patellar stabilization is the lateral retinaculum, which provides a restraint toward posterior rather than lateral (Figure 4). Cutting the lateral retinaculum would further decrease patellar stability in most cases.18,24-26
Figure 7.
We strongly recommend that physicians assess for all these osteochondral and soft-tissue abnormalities in patients with LPD.

Diagnostics

Physical Examination

It is recommended that the physician starts the examination by assessing the walking and standing patient while focusing on torsional malalignment of the lower extremities (increased antetorsion of the femur, increased external torsion of the tibia), which is often indicated by squinting patellae.8,27,28

Figure 8.
In addition, valgus knee alignment, increased foot pronation, and weakness of hip external rotators and hip abductors (Trendelenburg sign) are regularly observed in patients with LPD.29
Figure 9.
Beyond walking and standing, additional functional tests (eg, single-leg squat, single-leg balancing, step-down test) were suggested as reliably provoking these pathologic kinematics.30 It is also suggested that the patient be examined sitting with lower legs hanging. In many cases, patients who are asked to actively extend the leg with LPD present a so-called J sign, which means the patella moves laterally close to terminal knee extension (Figure 5). Examination continues with the patient supine. The physician uses the patella glide test to determine how far the patella can be translated toward lateral and medial. Grade 1 indicates the patella can be translated one-fourth of its width, and grade 4 indicates it can be translated its full width31 (Figure 6).
Figure 10.
The apprehension test is positive in the majority of patients with LPD and is performed in 30° knee flexion with relaxed quadriceps. The physician gently pushes the patella toward lateral. Avoidance or protective quadriceps contraction indicates a positive test.32,33 It is recommended that the physician forgo the Zohlen test (low specificity) and instead use the extension test, in which the patient tries to extend the leg against physician resistance at 0°, 30°, 60°, and 90°. The extension test provokes pain in the case of significant degeneration at the respective joint areas under contact pressure. The patient should also be examined in the prone position in order to assess for torsional deformities. With knees in 90° flexion, maximum external rotation and maximum internal rotation of the hips are determined on both sides at the same time (Figures 7A, 7B). Patients with significant internal rotation (>60°) and poor external rotation are suspected as having increased femoral antetorsion.

 

 

Imaging

Radiographs are the basis for each patient’s imaging analysis. For a patient with valgus or varus clinical appearance, a weight-bearing whole-leg radiograph is used to precisely assess the degree of deformity in the frontal plane. A true lateral radiograph (congruent posterior condyles) provides information about patellar height (patella alta/infera). Most indices that quantify patellar height use the tibia as reference (eg, tuberosity, anterior aspect of articulation surface).

Figure 11.
The Caton-Deschamps index measures the length of the articulating patella surface (A) and the distance from the most distal point of the patellar surface to the most anterior aspect of the articulating surface of the tibia (B); distance B divided by distance A yields the index, with values >1.2 indicating patella alta and values <0.6 indicating patella infera34 (Figure 8).
Figure 12.
The lateral radiograph should also be checked for trochlear dysplasia, indicated by the crossing sign, the trochlear bump, or both (Figure 9). A weight-bearing anteroposterior (eg, Schuss) radiograph, which provides information on accompanying degeneration of the tibiofemoral joint, should be performed, particularly for elderly patients.
Figure 13.

MRI is the gold standard for LPD diagnosis—it can be used to easily identify soft-tissue lesions and establish their patellar or femoral location (eg, MPFL rupture). MRI also provides information on potential pathologies of quadriceps tendon, patella tendon, and infrapatellar fat pad. Compared with radiographs, MRI is more sensitive in detecting osteochondral lesions in LPD.

Figure 14.
Furthermore, functional measurements (eg, patellar tilt, patellar shift) can be made on axial MRIs, as the posterior condyles provide a proper reference line. MRI also plays a key role in determining accompanying degenerative changes in patients with LPD and therefore helps distinguish between joint-preserving and prosthetic procedures.
Figure 15.
MRI also provides information on patellar height. In contrast to the radiographic patellar height assessment mentioned earlier, the patellotrochlear index of Biedert and Albrecht35 allows patellar height to be related to the proximal end of the trochlea.
Figure 16.
From a biomechanical point of view, it seems more appropriate to determine patellar height respective of the trochlea, the articulating partner of the patella.
Figure 17.
Further typical imaging parameters in LPD—such as TT-TG distance, femoral and tibial torsion values, and Dejour trochlear dysplasia—are also reliably shown with MRI. With lateral radiographs, MRI classifies trochlear dysplasia as type A (flatter than normal, with sulcus angle >145°), type B (flat), type C (convex), or type D (convex with supratrochlear spur and cliff) (Figures 10A-10D).

Treatment

MPFL Reconstruction

Isolated MPFL reconstruction is commonly regarded as a standard, straightforward procedure.

Figure 18.
However, some authors have reported a considerable complication rate.36 Most failures have been attributed to technical errors and inappropriate indications.
Figure 19.
The indication for isolated MPFL is regarded as inappropriate in patients with coexisting severe osseous pathologies, such as high-grade trochlear dysplasia and pathologic TT-TG distance.37,38
Figure 20.
We recommend against performing isolated MPFL reconstruction in patients with any of these conditions: TT-TG distance >20 mm; femoral anteversion >30°; type C or D trochlear dysplasia; severe patella alta; advanced patellofemoral cartilage degeneration; or tibiofemoral valgus >5°.
Figure 21.
With use of accurate indications and surgical technique, isolated MPFL reconstruction provides good outcomes in patients with LPD.39,40 MPFL reconstruction has been performed with a wide variety of surgical techniques (eg, graft type, single-bundle vs double-bundle, fixation type). Our preferred technique (double-bundle gracilis autograft with aperture fixation) is detailed in Figures 11 to 16.

Trochleoplasty

In cases of recurrent LPD or a flat or convex trochlea (Dejour type B, C, or D dysplasia), deepening trochleoplasty should be considered.

Figure 22.
Trochleoplasty is performed to reduce too prominent anterior bone stock and to increase conformity with the patella (concave groove), and to create a lateral trochlea facet as restraint against lateralizing quadriceps pull. Many authors have reported good clinical outcomes of trochleoplasty in patients with LPD caused by a dysplastic femoral trochlea.41-48
Table 1.
In many cases, MPFL reconstruction is added to trochleoplasty. Several authors have recommended against performing trochleoplasty in cases of open physis,49-52 which makes treatment of LPD in skeletally immature patients a special challenge, as trochlear dysplasia is often the key factor in failure of alternative procedures in the young.51 Another contraindication to trochleoplasty is severe cartilage degeneration. Our preferred surgical technique is described in detail in Figures 17 to 21.

Osteotomy

The most popular type of osteotomy in the setting of LPD is the transfer of the TT (TTT).

Table 2.
Many authors have reported good clinical outcomes with medializing TTT in patients with LPD and large TT-TG distances.53-57 Similarly, good outcomes have been found with distalizing TTT in patients with LPD and patella alta.58,59 We suggest routinely combining distal or medial TTT with MPFL reconstruction.60 TTT can be tailored to the patient’s pathology by combining medialization and distalization. Our preferred technique is to medialize the tuberosity so it ends with a TT-TG distance of at least 10 mm (avoid overcorrection).
Figure 23.

 

 

Derotational osteotomies of the femur (externally rotating) provide good outcomes in patients with LPD and associated torsional deformities,61-63 though the literature is incongruent with respect to whether rotational osteotomies of the femur should be performed at the proximal or distal aspect.64-67 In the majority of our LPD cases, we combine femoral derotation with MPFL reconstruction.

Treatment Algorithms

We suggest using different algorithms for primary LPD (Figure 22, Tables 1-2) and recurrent LPD (Figure 23).

Conclusion

In skeletally mature patients, LPD is sufficiently treated with modern versions of patellofemoral surgery. Comprehensive assessment for underlying pathology is paramount as preparation for developing an appropriate surgical plan for the patient.

 

Am J Orthop. 2017;46(2):E86-E96. Copyright Frontline Medical Communications Inc. 2017. All rights reserved.

 

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41. Neumann MV, Stalder M, Schuster AJ. Reconstructive surgery for patellofemoral joint incongruency. Knee Surg Sports Traumatol Arthrosc. 2016;24(3):873-878.

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43. Dejour D, Byn P, Ntagiopoulos PG. The Lyon’s sulcus-deepening trochleoplasty in previous unsuccessful patellofemoral surgery. Int Orthop. 2013;37(3):433-439.

44. Thaunat M, Bessiere C, Pujol N, Boisrenoult P, Beaufils P. Recession wedge trochleoplasty as an additional procedure in the surgical treatment of patellar instability with major trochlear dysplasia: early results. Orthop Traumatol Surg Res. 2011;97(8):833-845.

45. Utting MR, Mulford JS, Eldridge JD. A prospective evaluation of trochleoplasty for the treatment of patellofemoral dislocation and instability. J Bone Joint Surg Br. 2008;90(2):180-185.

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Michael C. Liebensteiner, MD, PhD
Florian Dirisamer, MD
Peter Balcarek, MD
Philip Schoettle, MD, PhD
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Michael C. Liebensteiner, MD, PhD
Florian Dirisamer, MD
Peter Balcarek, MD
Philip Schoettle, MD, PhD
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Take-Home Points

  • Lateral patella dislocation is sufficiently treated with modern versions of patellofemoral surgery.
  • Comprehensive assessment for underlying osseous pathology is paramount (torsional abnormalities of the femur or tibia, trochlea dysplasia, patella alta, etc).
  • In such cases, isolated medial patellofemoral ligament reconstructions will fail. Instead, the underlying osseous abnormalities must be addressed during concomitant procedures (derotational osteotomy, tibial tubercle transfer, trochleoplasty, etc).

The incidence of patellar instability is high, particularly in young females. In principle, cases of patellar instability can be classified as traumatic (dislocation is caused by external, often direct forces) or nontraumatic (anatomy predisposes to instability).1-4

Figure 1.
Because the vast majority of unstable patellae are unstable toward lateral and because instability is objective when the patella is fully dislocated, we use the term lateral patella dislocation (LPD) and refer to primary and recurrent LPD throughout this review.

Anatomy Predisposing to Patella Dislocation

Most patients present with specific anatomical factors that predispose to patellar instability (isolated or combined).

Figure 2.
These can be grossly categorized as osteochondral factors and soft-tissue factors.

Of the osteochondral factors, dysplasia of the femoral trochlea (trochlea groove [TG]) is most important. In healthy patients, the concave trochlea stabilizes the patella in knee flexion angles above 20°. In particular, the lateral facet of the trochlea plays a key role in withstanding the lateralizing quadriceps vector. The dysplastic trochlea, which has a flat or even a convex surface, destabilizes the patella (Figure 1). Moreover, patella alta is a pivotal factor in the development of LPD.

Figure 3.
A high-riding patella engages the femoral trochlea during higher degrees of knee flexion, making the patella very susceptible to dislocations when the knee is almost in extension.5,6 In addition, high femoral anteversion (increased femoral internal torsion) has been reported as contributing to the development of LPD. Internal torsion of the distal femur brings the TG more medial and therefore provokes a lateral shift of the patella relative to the femur (Figure 2).7-11 Valgus knee alignment is also common in patients with LPD.
Figure 4.
First, tibiofemoral valgus brings the tibial tuberosity (TT) more toward lateral and therefore increases the pull on the patella toward lateral. Second, when the deformity is at the distal femur, there is often a hypoplastic lateral condyle, which can contribute to LPD in knee flexion angles above 45°. Deformities in the frontal plane (valgus) and the transverse plane (increased internal torsion of the femur, increased external torsion of the proximal tibia) commonly increase the TT-TG distance. TT-TG distance is a radiographic parameter, taken from magnetic resonance imaging (MRI) or computed tomography, that summarizes important aspects of patellofemoral alignment and gives an impression of the amount of lateralizing force of the extensor apparatus (discussed later) (Figure 3).

The anteromedial soft tissue of the knee (retinaculum) has 3 layers, the second of which contains the
Figure 5.
medial patellofemoral ligament (MPFL).12,13 On the femoral side, the MPFL originates in direct proximity to the medial epicondyle and the adductor tubercle. The MPFL broadens toward the patella (V-shaped) and inserts at the superomedial border of the patella and the adjacent aspects of the quadriceps tendon.14-17
Figure 6.
It has been found to provide an important restraint against LPD.18-20 In primary LPD, the MPFL has been found ruptured or severely damaged in more than 90% of cases, most often near the femoral insertion.18,21-23 In patients with an elongated, insufficient MPFL, the patella may dislocate laterally without rupturing the MPFL. Another soft-tissue structure that contributes to patellar stabilization is the lateral retinaculum, which provides a restraint toward posterior rather than lateral (Figure 4). Cutting the lateral retinaculum would further decrease patellar stability in most cases.18,24-26
Figure 7.
We strongly recommend that physicians assess for all these osteochondral and soft-tissue abnormalities in patients with LPD.

Diagnostics

Physical Examination

It is recommended that the physician starts the examination by assessing the walking and standing patient while focusing on torsional malalignment of the lower extremities (increased antetorsion of the femur, increased external torsion of the tibia), which is often indicated by squinting patellae.8,27,28

Figure 8.
In addition, valgus knee alignment, increased foot pronation, and weakness of hip external rotators and hip abductors (Trendelenburg sign) are regularly observed in patients with LPD.29
Figure 9.
Beyond walking and standing, additional functional tests (eg, single-leg squat, single-leg balancing, step-down test) were suggested as reliably provoking these pathologic kinematics.30 It is also suggested that the patient be examined sitting with lower legs hanging. In many cases, patients who are asked to actively extend the leg with LPD present a so-called J sign, which means the patella moves laterally close to terminal knee extension (Figure 5). Examination continues with the patient supine. The physician uses the patella glide test to determine how far the patella can be translated toward lateral and medial. Grade 1 indicates the patella can be translated one-fourth of its width, and grade 4 indicates it can be translated its full width31 (Figure 6).
Figure 10.
The apprehension test is positive in the majority of patients with LPD and is performed in 30° knee flexion with relaxed quadriceps. The physician gently pushes the patella toward lateral. Avoidance or protective quadriceps contraction indicates a positive test.32,33 It is recommended that the physician forgo the Zohlen test (low specificity) and instead use the extension test, in which the patient tries to extend the leg against physician resistance at 0°, 30°, 60°, and 90°. The extension test provokes pain in the case of significant degeneration at the respective joint areas under contact pressure. The patient should also be examined in the prone position in order to assess for torsional deformities. With knees in 90° flexion, maximum external rotation and maximum internal rotation of the hips are determined on both sides at the same time (Figures 7A, 7B). Patients with significant internal rotation (>60°) and poor external rotation are suspected as having increased femoral antetorsion.

 

 

Imaging

Radiographs are the basis for each patient’s imaging analysis. For a patient with valgus or varus clinical appearance, a weight-bearing whole-leg radiograph is used to precisely assess the degree of deformity in the frontal plane. A true lateral radiograph (congruent posterior condyles) provides information about patellar height (patella alta/infera). Most indices that quantify patellar height use the tibia as reference (eg, tuberosity, anterior aspect of articulation surface).

Figure 11.
The Caton-Deschamps index measures the length of the articulating patella surface (A) and the distance from the most distal point of the patellar surface to the most anterior aspect of the articulating surface of the tibia (B); distance B divided by distance A yields the index, with values >1.2 indicating patella alta and values <0.6 indicating patella infera34 (Figure 8).
Figure 12.
The lateral radiograph should also be checked for trochlear dysplasia, indicated by the crossing sign, the trochlear bump, or both (Figure 9). A weight-bearing anteroposterior (eg, Schuss) radiograph, which provides information on accompanying degeneration of the tibiofemoral joint, should be performed, particularly for elderly patients.
Figure 13.

MRI is the gold standard for LPD diagnosis—it can be used to easily identify soft-tissue lesions and establish their patellar or femoral location (eg, MPFL rupture). MRI also provides information on potential pathologies of quadriceps tendon, patella tendon, and infrapatellar fat pad. Compared with radiographs, MRI is more sensitive in detecting osteochondral lesions in LPD.

Figure 14.
Furthermore, functional measurements (eg, patellar tilt, patellar shift) can be made on axial MRIs, as the posterior condyles provide a proper reference line. MRI also plays a key role in determining accompanying degenerative changes in patients with LPD and therefore helps distinguish between joint-preserving and prosthetic procedures.
Figure 15.
MRI also provides information on patellar height. In contrast to the radiographic patellar height assessment mentioned earlier, the patellotrochlear index of Biedert and Albrecht35 allows patellar height to be related to the proximal end of the trochlea.
Figure 16.
From a biomechanical point of view, it seems more appropriate to determine patellar height respective of the trochlea, the articulating partner of the patella.
Figure 17.
Further typical imaging parameters in LPD—such as TT-TG distance, femoral and tibial torsion values, and Dejour trochlear dysplasia—are also reliably shown with MRI. With lateral radiographs, MRI classifies trochlear dysplasia as type A (flatter than normal, with sulcus angle >145°), type B (flat), type C (convex), or type D (convex with supratrochlear spur and cliff) (Figures 10A-10D).

Treatment

MPFL Reconstruction

Isolated MPFL reconstruction is commonly regarded as a standard, straightforward procedure.

Figure 18.
However, some authors have reported a considerable complication rate.36 Most failures have been attributed to technical errors and inappropriate indications.
Figure 19.
The indication for isolated MPFL is regarded as inappropriate in patients with coexisting severe osseous pathologies, such as high-grade trochlear dysplasia and pathologic TT-TG distance.37,38
Figure 20.
We recommend against performing isolated MPFL reconstruction in patients with any of these conditions: TT-TG distance >20 mm; femoral anteversion >30°; type C or D trochlear dysplasia; severe patella alta; advanced patellofemoral cartilage degeneration; or tibiofemoral valgus >5°.
Figure 21.
With use of accurate indications and surgical technique, isolated MPFL reconstruction provides good outcomes in patients with LPD.39,40 MPFL reconstruction has been performed with a wide variety of surgical techniques (eg, graft type, single-bundle vs double-bundle, fixation type). Our preferred technique (double-bundle gracilis autograft with aperture fixation) is detailed in Figures 11 to 16.

Trochleoplasty

In cases of recurrent LPD or a flat or convex trochlea (Dejour type B, C, or D dysplasia), deepening trochleoplasty should be considered.

Figure 22.
Trochleoplasty is performed to reduce too prominent anterior bone stock and to increase conformity with the patella (concave groove), and to create a lateral trochlea facet as restraint against lateralizing quadriceps pull. Many authors have reported good clinical outcomes of trochleoplasty in patients with LPD caused by a dysplastic femoral trochlea.41-48
Table 1.
In many cases, MPFL reconstruction is added to trochleoplasty. Several authors have recommended against performing trochleoplasty in cases of open physis,49-52 which makes treatment of LPD in skeletally immature patients a special challenge, as trochlear dysplasia is often the key factor in failure of alternative procedures in the young.51 Another contraindication to trochleoplasty is severe cartilage degeneration. Our preferred surgical technique is described in detail in Figures 17 to 21.

Osteotomy

The most popular type of osteotomy in the setting of LPD is the transfer of the TT (TTT).

Table 2.
Many authors have reported good clinical outcomes with medializing TTT in patients with LPD and large TT-TG distances.53-57 Similarly, good outcomes have been found with distalizing TTT in patients with LPD and patella alta.58,59 We suggest routinely combining distal or medial TTT with MPFL reconstruction.60 TTT can be tailored to the patient’s pathology by combining medialization and distalization. Our preferred technique is to medialize the tuberosity so it ends with a TT-TG distance of at least 10 mm (avoid overcorrection).
Figure 23.

 

 

Derotational osteotomies of the femur (externally rotating) provide good outcomes in patients with LPD and associated torsional deformities,61-63 though the literature is incongruent with respect to whether rotational osteotomies of the femur should be performed at the proximal or distal aspect.64-67 In the majority of our LPD cases, we combine femoral derotation with MPFL reconstruction.

Treatment Algorithms

We suggest using different algorithms for primary LPD (Figure 22, Tables 1-2) and recurrent LPD (Figure 23).

Conclusion

In skeletally mature patients, LPD is sufficiently treated with modern versions of patellofemoral surgery. Comprehensive assessment for underlying pathology is paramount as preparation for developing an appropriate surgical plan for the patient.

 

Am J Orthop. 2017;46(2):E86-E96. Copyright Frontline Medical Communications Inc. 2017. All rights reserved.

 

Take-Home Points

  • Lateral patella dislocation is sufficiently treated with modern versions of patellofemoral surgery.
  • Comprehensive assessment for underlying osseous pathology is paramount (torsional abnormalities of the femur or tibia, trochlea dysplasia, patella alta, etc).
  • In such cases, isolated medial patellofemoral ligament reconstructions will fail. Instead, the underlying osseous abnormalities must be addressed during concomitant procedures (derotational osteotomy, tibial tubercle transfer, trochleoplasty, etc).

The incidence of patellar instability is high, particularly in young females. In principle, cases of patellar instability can be classified as traumatic (dislocation is caused by external, often direct forces) or nontraumatic (anatomy predisposes to instability).1-4

Figure 1.
Because the vast majority of unstable patellae are unstable toward lateral and because instability is objective when the patella is fully dislocated, we use the term lateral patella dislocation (LPD) and refer to primary and recurrent LPD throughout this review.

Anatomy Predisposing to Patella Dislocation

Most patients present with specific anatomical factors that predispose to patellar instability (isolated or combined).

Figure 2.
These can be grossly categorized as osteochondral factors and soft-tissue factors.

Of the osteochondral factors, dysplasia of the femoral trochlea (trochlea groove [TG]) is most important. In healthy patients, the concave trochlea stabilizes the patella in knee flexion angles above 20°. In particular, the lateral facet of the trochlea plays a key role in withstanding the lateralizing quadriceps vector. The dysplastic trochlea, which has a flat or even a convex surface, destabilizes the patella (Figure 1). Moreover, patella alta is a pivotal factor in the development of LPD.

Figure 3.
A high-riding patella engages the femoral trochlea during higher degrees of knee flexion, making the patella very susceptible to dislocations when the knee is almost in extension.5,6 In addition, high femoral anteversion (increased femoral internal torsion) has been reported as contributing to the development of LPD. Internal torsion of the distal femur brings the TG more medial and therefore provokes a lateral shift of the patella relative to the femur (Figure 2).7-11 Valgus knee alignment is also common in patients with LPD.
Figure 4.
First, tibiofemoral valgus brings the tibial tuberosity (TT) more toward lateral and therefore increases the pull on the patella toward lateral. Second, when the deformity is at the distal femur, there is often a hypoplastic lateral condyle, which can contribute to LPD in knee flexion angles above 45°. Deformities in the frontal plane (valgus) and the transverse plane (increased internal torsion of the femur, increased external torsion of the proximal tibia) commonly increase the TT-TG distance. TT-TG distance is a radiographic parameter, taken from magnetic resonance imaging (MRI) or computed tomography, that summarizes important aspects of patellofemoral alignment and gives an impression of the amount of lateralizing force of the extensor apparatus (discussed later) (Figure 3).

The anteromedial soft tissue of the knee (retinaculum) has 3 layers, the second of which contains the
Figure 5.
medial patellofemoral ligament (MPFL).12,13 On the femoral side, the MPFL originates in direct proximity to the medial epicondyle and the adductor tubercle. The MPFL broadens toward the patella (V-shaped) and inserts at the superomedial border of the patella and the adjacent aspects of the quadriceps tendon.14-17
Figure 6.
It has been found to provide an important restraint against LPD.18-20 In primary LPD, the MPFL has been found ruptured or severely damaged in more than 90% of cases, most often near the femoral insertion.18,21-23 In patients with an elongated, insufficient MPFL, the patella may dislocate laterally without rupturing the MPFL. Another soft-tissue structure that contributes to patellar stabilization is the lateral retinaculum, which provides a restraint toward posterior rather than lateral (Figure 4). Cutting the lateral retinaculum would further decrease patellar stability in most cases.18,24-26
Figure 7.
We strongly recommend that physicians assess for all these osteochondral and soft-tissue abnormalities in patients with LPD.

Diagnostics

Physical Examination

It is recommended that the physician starts the examination by assessing the walking and standing patient while focusing on torsional malalignment of the lower extremities (increased antetorsion of the femur, increased external torsion of the tibia), which is often indicated by squinting patellae.8,27,28

Figure 8.
In addition, valgus knee alignment, increased foot pronation, and weakness of hip external rotators and hip abductors (Trendelenburg sign) are regularly observed in patients with LPD.29
Figure 9.
Beyond walking and standing, additional functional tests (eg, single-leg squat, single-leg balancing, step-down test) were suggested as reliably provoking these pathologic kinematics.30 It is also suggested that the patient be examined sitting with lower legs hanging. In many cases, patients who are asked to actively extend the leg with LPD present a so-called J sign, which means the patella moves laterally close to terminal knee extension (Figure 5). Examination continues with the patient supine. The physician uses the patella glide test to determine how far the patella can be translated toward lateral and medial. Grade 1 indicates the patella can be translated one-fourth of its width, and grade 4 indicates it can be translated its full width31 (Figure 6).
Figure 10.
The apprehension test is positive in the majority of patients with LPD and is performed in 30° knee flexion with relaxed quadriceps. The physician gently pushes the patella toward lateral. Avoidance or protective quadriceps contraction indicates a positive test.32,33 It is recommended that the physician forgo the Zohlen test (low specificity) and instead use the extension test, in which the patient tries to extend the leg against physician resistance at 0°, 30°, 60°, and 90°. The extension test provokes pain in the case of significant degeneration at the respective joint areas under contact pressure. The patient should also be examined in the prone position in order to assess for torsional deformities. With knees in 90° flexion, maximum external rotation and maximum internal rotation of the hips are determined on both sides at the same time (Figures 7A, 7B). Patients with significant internal rotation (>60°) and poor external rotation are suspected as having increased femoral antetorsion.

 

 

Imaging

Radiographs are the basis for each patient’s imaging analysis. For a patient with valgus or varus clinical appearance, a weight-bearing whole-leg radiograph is used to precisely assess the degree of deformity in the frontal plane. A true lateral radiograph (congruent posterior condyles) provides information about patellar height (patella alta/infera). Most indices that quantify patellar height use the tibia as reference (eg, tuberosity, anterior aspect of articulation surface).

Figure 11.
The Caton-Deschamps index measures the length of the articulating patella surface (A) and the distance from the most distal point of the patellar surface to the most anterior aspect of the articulating surface of the tibia (B); distance B divided by distance A yields the index, with values >1.2 indicating patella alta and values <0.6 indicating patella infera34 (Figure 8).
Figure 12.
The lateral radiograph should also be checked for trochlear dysplasia, indicated by the crossing sign, the trochlear bump, or both (Figure 9). A weight-bearing anteroposterior (eg, Schuss) radiograph, which provides information on accompanying degeneration of the tibiofemoral joint, should be performed, particularly for elderly patients.
Figure 13.

MRI is the gold standard for LPD diagnosis—it can be used to easily identify soft-tissue lesions and establish their patellar or femoral location (eg, MPFL rupture). MRI also provides information on potential pathologies of quadriceps tendon, patella tendon, and infrapatellar fat pad. Compared with radiographs, MRI is more sensitive in detecting osteochondral lesions in LPD.

Figure 14.
Furthermore, functional measurements (eg, patellar tilt, patellar shift) can be made on axial MRIs, as the posterior condyles provide a proper reference line. MRI also plays a key role in determining accompanying degenerative changes in patients with LPD and therefore helps distinguish between joint-preserving and prosthetic procedures.
Figure 15.
MRI also provides information on patellar height. In contrast to the radiographic patellar height assessment mentioned earlier, the patellotrochlear index of Biedert and Albrecht35 allows patellar height to be related to the proximal end of the trochlea.
Figure 16.
From a biomechanical point of view, it seems more appropriate to determine patellar height respective of the trochlea, the articulating partner of the patella.
Figure 17.
Further typical imaging parameters in LPD—such as TT-TG distance, femoral and tibial torsion values, and Dejour trochlear dysplasia—are also reliably shown with MRI. With lateral radiographs, MRI classifies trochlear dysplasia as type A (flatter than normal, with sulcus angle >145°), type B (flat), type C (convex), or type D (convex with supratrochlear spur and cliff) (Figures 10A-10D).

Treatment

MPFL Reconstruction

Isolated MPFL reconstruction is commonly regarded as a standard, straightforward procedure.

Figure 18.
However, some authors have reported a considerable complication rate.36 Most failures have been attributed to technical errors and inappropriate indications.
Figure 19.
The indication for isolated MPFL is regarded as inappropriate in patients with coexisting severe osseous pathologies, such as high-grade trochlear dysplasia and pathologic TT-TG distance.37,38
Figure 20.
We recommend against performing isolated MPFL reconstruction in patients with any of these conditions: TT-TG distance >20 mm; femoral anteversion >30°; type C or D trochlear dysplasia; severe patella alta; advanced patellofemoral cartilage degeneration; or tibiofemoral valgus >5°.
Figure 21.
With use of accurate indications and surgical technique, isolated MPFL reconstruction provides good outcomes in patients with LPD.39,40 MPFL reconstruction has been performed with a wide variety of surgical techniques (eg, graft type, single-bundle vs double-bundle, fixation type). Our preferred technique (double-bundle gracilis autograft with aperture fixation) is detailed in Figures 11 to 16.

Trochleoplasty

In cases of recurrent LPD or a flat or convex trochlea (Dejour type B, C, or D dysplasia), deepening trochleoplasty should be considered.

Figure 22.
Trochleoplasty is performed to reduce too prominent anterior bone stock and to increase conformity with the patella (concave groove), and to create a lateral trochlea facet as restraint against lateralizing quadriceps pull. Many authors have reported good clinical outcomes of trochleoplasty in patients with LPD caused by a dysplastic femoral trochlea.41-48
Table 1.
In many cases, MPFL reconstruction is added to trochleoplasty. Several authors have recommended against performing trochleoplasty in cases of open physis,49-52 which makes treatment of LPD in skeletally immature patients a special challenge, as trochlear dysplasia is often the key factor in failure of alternative procedures in the young.51 Another contraindication to trochleoplasty is severe cartilage degeneration. Our preferred surgical technique is described in detail in Figures 17 to 21.

Osteotomy

The most popular type of osteotomy in the setting of LPD is the transfer of the TT (TTT).

Table 2.
Many authors have reported good clinical outcomes with medializing TTT in patients with LPD and large TT-TG distances.53-57 Similarly, good outcomes have been found with distalizing TTT in patients with LPD and patella alta.58,59 We suggest routinely combining distal or medial TTT with MPFL reconstruction.60 TTT can be tailored to the patient’s pathology by combining medialization and distalization. Our preferred technique is to medialize the tuberosity so it ends with a TT-TG distance of at least 10 mm (avoid overcorrection).
Figure 23.

 

 

Derotational osteotomies of the femur (externally rotating) provide good outcomes in patients with LPD and associated torsional deformities,61-63 though the literature is incongruent with respect to whether rotational osteotomies of the femur should be performed at the proximal or distal aspect.64-67 In the majority of our LPD cases, we combine femoral derotation with MPFL reconstruction.

Treatment Algorithms

We suggest using different algorithms for primary LPD (Figure 22, Tables 1-2) and recurrent LPD (Figure 23).

Conclusion

In skeletally mature patients, LPD is sufficiently treated with modern versions of patellofemoral surgery. Comprehensive assessment for underlying pathology is paramount as preparation for developing an appropriate surgical plan for the patient.

 

Am J Orthop. 2017;46(2):E86-E96. Copyright Frontline Medical Communications Inc. 2017. All rights reserved.

 

References

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41. Neumann MV, Stalder M, Schuster AJ. Reconstructive surgery for patellofemoral joint incongruency. Knee Surg Sports Traumatol Arthrosc. 2016;24(3):873-878.

42. Banke IJ, Kohn LM, Meidinger G, et al. Combined trochleoplasty and MPFL reconstruction for treatment of chronic patellofemoral instability: a prospective minimum 2-year follow-up study. Knee Surg Sports Traumatol Arthrosc. 2014;22(11):2591-2598.

43. Dejour D, Byn P, Ntagiopoulos PG. The Lyon’s sulcus-deepening trochleoplasty in previous unsuccessful patellofemoral surgery. Int Orthop. 2013;37(3):433-439.

44. Thaunat M, Bessiere C, Pujol N, Boisrenoult P, Beaufils P. Recession wedge trochleoplasty as an additional procedure in the surgical treatment of patellar instability with major trochlear dysplasia: early results. Orthop Traumatol Surg Res. 2011;97(8):833-845.

45. Utting MR, Mulford JS, Eldridge JD. A prospective evaluation of trochleoplasty for the treatment of patellofemoral dislocation and instability. J Bone Joint Surg Br. 2008;90(2):180-185.

46. Blønd L, Haugegaard M. Combined arthroscopic deepening trochleoplasty and reconstruction of the medial patellofemoral ligament for patients with recurrent patella dislocation and trochlear dysplasia. Knee Surg Sports Traumatol Arthrosc. 2014;22(10):2484-2490.

47. Nelitz M, Dreyhaupt J, Lippacher S. Combined trochleoplasty and medial patellofemoral ligament reconstruction for recurrent patellar dislocations in severe trochlear dysplasia: a minimum 2-year follow-up study. Am J Sports Med. 2013;41(5):1005-1012.

48. Ntagiopoulos PG, Byn P, Dejour D. Midterm results of comprehensive surgical reconstruction including sulcus-deepening trochleoplasty in recurrent patellar dislocations with high-grade trochlear dysplasia. Am J Sports Med. 2013;41(5):998-1004.

49. Biedert R. Trochleoplasty—simple or tricky? Knee. 2014;21(6):1297-1298.

50. Ntagiopoulos PG, Dejour D. Current concepts on trochleoplasty procedures for the surgical treatment of trochlear dysplasia. Knee Surg Sports Traumatol Arthrosc. 2014;22(10):2531-2539.

51. Nelitz M, Theile M, Dornacher D, Wölfle J, Reichel H, Lippacher S. Analysis of failed surgery for patellar instability in children with open growth plates. Knee Surg Sports Traumatol Arthrosc. 2012;20(5):822-828.

52. Schöttle PB, Fucentese SF, Pfirrmann C, Bereiter H, Romero J. Trochleaplasty for patellar instability due to trochlear dysplasia: a minimum 2-year clinical and radiological follow-up of 19 knees. Acta Orthop. 2005;76(5):693-698.

53. Longo UG, Rizzello G, Ciuffreda M, et al. Elmslie-Trillat, Maquet, Fulkerson, Roux Goldthwait, and other distal realignment procedures for the management of patellar dislocation: systematic review and quantitative synthesis of the literature. Arthroscopy. 2016;32(5):929-943.

54. Barber FA, McGarry JE. Elmslie-Trillat procedure for the treatment of recurrent patellar instability. Arthroscopy. 2008;24(1):77-81.

55. Karataglis D, Green MA, Learmonth DJ. Functional outcome following modified Elmslie-Trillat procedure. Knee. 2006;13(6):464-468.

56. Kumar A, Jones S, Bickerstaff DR, Smith TW. A functional evaluation of the modified Elmslie-Trillat procedure for patello-femoral dysfunction. Knee. 2001;8(4):287-292.

57. Nakagawa K, Wada Y, Minamide M, Tsuchiya A, Moriya H. Deterioration of long-term clinical results after the Elmslie-Trillat procedure for dislocation of the patella. J Bone Joint Surg Br. 2002;84(6):861-864.

58. Magnussen RA, De Simone V, Lustig S, Neyret P, Flanigan DC. Treatment of patella alta in patients with episodic patellar dislocation: a systematic review. Knee Surg Sports Traumatol Arthrosc. 2014;22(10):2545-2550.

59. Mayer C, Magnussen RA, Servien E, et al. Patellar tendon tenodesis in association with tibial tubercle distalization for the treatment of episodic patellar dislocation with patella alta. Am J Sports Med. 2012;40(2):346-351.

60. Burnham JM, Howard JS, Hayes CB, Lattermann C. Medial patellofemoral ligament reconstruction with concomitant tibial tubercle transfer: a systematic review of outcomes and complications. Arthroscopy. 2016;32(6):1185-1195.

61. Dickschas J, Harrer J, Pfefferkorn R, Strecker W. Operative treatment of patellofemoral maltracking with torsional osteotomy. Arch Orthop Trauma Surg. 2012;132(3):289-298.

62. Nelitz M, Dreyhaupt J, Williams SR, Dornacher D. Combined supracondylar femoral derotation osteotomy and patellofemoral ligament reconstruction for recurrent patellar dislocation and severe femoral anteversion syndrome: surgical technique and clinical outcome. Int Orthop. 2015;39(12):2355-2362.

63. Strecker W, Dickschas J. Torsional osteotomy: operative treatment of patellofemoral maltracking [in German]. Oper Orthop Traumatol. 2015;27(6):505-524.

 

 

64. Bruce WD, Stevens PM. Surgical correction of miserable malalignment syndrome. J Pediatr Orthop. 2004;24(4):392-396.

65. Delgado ED, Schoenecker PL, Rich MM, Capelli AM. Treatment of severe torsional malalignment syndrome. J Pediatr Orthop. 1996;16(4):484-488.

66. Dickschas J, Harrer J, Reuter B, Schwitulla J, Strecker W. Torsional osteotomies of the femur. J Orthop Res. 2015;33(3):318-324.

67. Stevens PM, Gililland JM, Anderson LA, Mickelson JB, Nielson J, Klatt JW. Success of torsional correction surgery after failed surgeries for patellofemoral pain and instability. Strategies Trauma Limb Reconstr. 2014;9(1):5-12.

68. Balcarek P, Oberthür S, Hopfensitz S, et al. Which patellae are likely to redislocate? Knee Surg Sports Traumatol Arthrosc. 2014;22(10):2308-2314.

69. Jaquith BP, Parikh SN. Predictors of recurrent patellar instability in children and adolescents after first-time dislocation [published online October 21, 2015]. J Pediatr Orthop. doi:10.1097/BPO.0000000000000674.

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32. Fairbank HA. Internal derangement of the knee in children and adolescents: (Section of Orthopaedics). Proc R Soc Med. 1937;30(4):427-432.

33. Hughston JC. Subluxation of the patella. J Bone Joint Surg Am. 1968;50(5):1003-1026.

34. Caton JH, Dejour D. Tibial tubercle osteotomy in patello-femoral instability and in patellar height abnormality. Int Orthop. 2010;34(2):305-309.

35. Biedert RM, Albrecht S. The patellotrochlear index: a new index for assessing patellar height. Knee Surg Sports Traumatol Arthrosc. 2006;14(8):707-712.

36. Shah JN, Howard JS, Flanigan DC, Brophy RH, Carey JL, Lattermann C. A systematic review of complications and failures associated with medial patellofemoral ligament reconstruction for recurrent patellar dislocation. Am J Sports Med. 2012;40(8):1916-1923.

37. Hopper GP, Leach WJ, Rooney BP, Walker CR, Blyth MJ. Does degree of trochlear dysplasia and position of femoral tunnel influence outcome after medial patellofemoral ligament reconstruction? Am J Sports Med. 2014;42(3):716-722.

38. Wagner D, Pfalzer F, Hingelbaum S, Huth J, Mauch F, Bauer G. The influence of risk factors on clinical outcomes following anatomical medial patellofemoral ligament (MPFL) reconstruction using the gracilis tendon. Knee Surg Sports Traumatol Arthrosc. 2013;21(2):318-324.

39. Mackay ND, Smith NA, Parsons N, Spalding T, Thompson P, Sprowson AP. Medial patellofemoral ligament reconstruction for patellar dislocation: a systematic review. Orthop J Sports Med. 2014;2(8):2325967114544021.

40. Stupay KL, Swart E, Shubin Stein BE. Widespread implementation of medial patellofemoral ligament reconstruction for recurrent patellar instability maintains functional outcomes at midterm to long-term follow-up while decreasing complication rates: a systematic review. Arthroscopy. 2015;31(7):1372-1380.

41. Neumann MV, Stalder M, Schuster AJ. Reconstructive surgery for patellofemoral joint incongruency. Knee Surg Sports Traumatol Arthrosc. 2016;24(3):873-878.

42. Banke IJ, Kohn LM, Meidinger G, et al. Combined trochleoplasty and MPFL reconstruction for treatment of chronic patellofemoral instability: a prospective minimum 2-year follow-up study. Knee Surg Sports Traumatol Arthrosc. 2014;22(11):2591-2598.

43. Dejour D, Byn P, Ntagiopoulos PG. The Lyon’s sulcus-deepening trochleoplasty in previous unsuccessful patellofemoral surgery. Int Orthop. 2013;37(3):433-439.

44. Thaunat M, Bessiere C, Pujol N, Boisrenoult P, Beaufils P. Recession wedge trochleoplasty as an additional procedure in the surgical treatment of patellar instability with major trochlear dysplasia: early results. Orthop Traumatol Surg Res. 2011;97(8):833-845.

45. Utting MR, Mulford JS, Eldridge JD. A prospective evaluation of trochleoplasty for the treatment of patellofemoral dislocation and instability. J Bone Joint Surg Br. 2008;90(2):180-185.

46. Blønd L, Haugegaard M. Combined arthroscopic deepening trochleoplasty and reconstruction of the medial patellofemoral ligament for patients with recurrent patella dislocation and trochlear dysplasia. Knee Surg Sports Traumatol Arthrosc. 2014;22(10):2484-2490.

47. Nelitz M, Dreyhaupt J, Lippacher S. Combined trochleoplasty and medial patellofemoral ligament reconstruction for recurrent patellar dislocations in severe trochlear dysplasia: a minimum 2-year follow-up study. Am J Sports Med. 2013;41(5):1005-1012.

48. Ntagiopoulos PG, Byn P, Dejour D. Midterm results of comprehensive surgical reconstruction including sulcus-deepening trochleoplasty in recurrent patellar dislocations with high-grade trochlear dysplasia. Am J Sports Med. 2013;41(5):998-1004.

49. Biedert R. Trochleoplasty—simple or tricky? Knee. 2014;21(6):1297-1298.

50. Ntagiopoulos PG, Dejour D. Current concepts on trochleoplasty procedures for the surgical treatment of trochlear dysplasia. Knee Surg Sports Traumatol Arthrosc. 2014;22(10):2531-2539.

51. Nelitz M, Theile M, Dornacher D, Wölfle J, Reichel H, Lippacher S. Analysis of failed surgery for patellar instability in children with open growth plates. Knee Surg Sports Traumatol Arthrosc. 2012;20(5):822-828.

52. Schöttle PB, Fucentese SF, Pfirrmann C, Bereiter H, Romero J. Trochleaplasty for patellar instability due to trochlear dysplasia: a minimum 2-year clinical and radiological follow-up of 19 knees. Acta Orthop. 2005;76(5):693-698.

53. Longo UG, Rizzello G, Ciuffreda M, et al. Elmslie-Trillat, Maquet, Fulkerson, Roux Goldthwait, and other distal realignment procedures for the management of patellar dislocation: systematic review and quantitative synthesis of the literature. Arthroscopy. 2016;32(5):929-943.

54. Barber FA, McGarry JE. Elmslie-Trillat procedure for the treatment of recurrent patellar instability. Arthroscopy. 2008;24(1):77-81.

55. Karataglis D, Green MA, Learmonth DJ. Functional outcome following modified Elmslie-Trillat procedure. Knee. 2006;13(6):464-468.

56. Kumar A, Jones S, Bickerstaff DR, Smith TW. A functional evaluation of the modified Elmslie-Trillat procedure for patello-femoral dysfunction. Knee. 2001;8(4):287-292.

57. Nakagawa K, Wada Y, Minamide M, Tsuchiya A, Moriya H. Deterioration of long-term clinical results after the Elmslie-Trillat procedure for dislocation of the patella. J Bone Joint Surg Br. 2002;84(6):861-864.

58. Magnussen RA, De Simone V, Lustig S, Neyret P, Flanigan DC. Treatment of patella alta in patients with episodic patellar dislocation: a systematic review. Knee Surg Sports Traumatol Arthrosc. 2014;22(10):2545-2550.

59. Mayer C, Magnussen RA, Servien E, et al. Patellar tendon tenodesis in association with tibial tubercle distalization for the treatment of episodic patellar dislocation with patella alta. Am J Sports Med. 2012;40(2):346-351.

60. Burnham JM, Howard JS, Hayes CB, Lattermann C. Medial patellofemoral ligament reconstruction with concomitant tibial tubercle transfer: a systematic review of outcomes and complications. Arthroscopy. 2016;32(6):1185-1195.

61. Dickschas J, Harrer J, Pfefferkorn R, Strecker W. Operative treatment of patellofemoral maltracking with torsional osteotomy. Arch Orthop Trauma Surg. 2012;132(3):289-298.

62. Nelitz M, Dreyhaupt J, Williams SR, Dornacher D. Combined supracondylar femoral derotation osteotomy and patellofemoral ligament reconstruction for recurrent patellar dislocation and severe femoral anteversion syndrome: surgical technique and clinical outcome. Int Orthop. 2015;39(12):2355-2362.

63. Strecker W, Dickschas J. Torsional osteotomy: operative treatment of patellofemoral maltracking [in German]. Oper Orthop Traumatol. 2015;27(6):505-524.

 

 

64. Bruce WD, Stevens PM. Surgical correction of miserable malalignment syndrome. J Pediatr Orthop. 2004;24(4):392-396.

65. Delgado ED, Schoenecker PL, Rich MM, Capelli AM. Treatment of severe torsional malalignment syndrome. J Pediatr Orthop. 1996;16(4):484-488.

66. Dickschas J, Harrer J, Reuter B, Schwitulla J, Strecker W. Torsional osteotomies of the femur. J Orthop Res. 2015;33(3):318-324.

67. Stevens PM, Gililland JM, Anderson LA, Mickelson JB, Nielson J, Klatt JW. Success of torsional correction surgery after failed surgeries for patellofemoral pain and instability. Strategies Trauma Limb Reconstr. 2014;9(1):5-12.

68. Balcarek P, Oberthür S, Hopfensitz S, et al. Which patellae are likely to redislocate? Knee Surg Sports Traumatol Arthrosc. 2014;22(10):2308-2314.

69. Jaquith BP, Parikh SN. Predictors of recurrent patellar instability in children and adolescents after first-time dislocation [published online October 21, 2015]. J Pediatr Orthop. doi:10.1097/BPO.0000000000000674.

Issue
The American Journal of Orthopedics - 46(2)
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The American Journal of Orthopedics - 46(2)
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E86-E96
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MDedge Surgery
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Guidelines for Treatment of Lateral Patella Dislocations in Skeletally Mature Patients
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Guidelines for Treatment of Lateral Patella Dislocations in Skeletally Mature Patients
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