Patellar Instability. Michael J. Alaia, M.D., Randy M. Cohn, M.D., and Eric J. Strauss, M.D. Anatomy and Biomechanics

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1 6 Bulletin of the Hospital for Joint Diseases 2014;72(1):6-17 Patellar Instability Michael J. Alaia, M.D., Randy M. Cohn, M.D., and Eric J. Strauss, M.D. Abstract Patellar instability is a complex pathoanatomical phenomenon that requires an intricate understanding in order to properly treat patients. Often, the etiology of this entity is multifactorial, combining a series of physiologic and anatomic variables. A thorough history and physical as well as correct radiographic examinations are critical in both establishing the diagnosis and instituting the correct treatment. Non-operative management of recurrent instability has a high failure rate. Current operative techniques have been shown to be instrumental in correcting anatomic abnormalities, reducing symptoms of instability, and giving patients an appropriate chance of returning to their previous level of activity. Recurrent patellar instability is a relatively rare phenomenon in which patients suffer either from multiple patellar dislocations, subluxations, or the feeling of instability. It is most common in young, active individuals. 1-3 Atkin reported the risk of patellar dislocation to be 7 per 100,000; however, this number increases to 31 per 100,000 in persons in their second decade of life. 1 Hawkins noted in the 1980s that at least 30% to 50% of patients that sustain a primary patellar dislocation will go on to have future episodes of instability or anterior knee pain. 3 Fithian reported that patients presenting with recurrent instability were more likely to be female and older than patients presenting with first-time dislocations. Additionally, 49% of patients had future episodes of instability if they had a history of recurrent dislocation or subluxation, compared Michael J. Alaia, M.D., Randy M. Cohn, M.D., and Eric J. Strauss, M.D., are in the Department of Orthopaedic Surgery, NYU Langone Medical Center, Hospital for Joint Diseases, New York, New York. Correspondence: Michael J. Alaia, M.D., 240 East 39th Street, Apt 7J, New York, New York 10016; alaiamj@gmail.com. with 17% of first-time dislocators. This represents a 7 times higher risk of future events in patients with a history of recurrent instability. 2 Multiple anatomical factors have been implicated in the etiology of recurrent instability, and an understanding of the interplay of these factors is critical when establishing a treatment plan. Anatomy and Biomechanics The anatomy of the lower extremity contributes many factors in the pathology of patellofemoral instability (PFI). The patella is constrained in the patellofemoral joint during the flexion cycle by a complex interplay between bony restraints and other primary stabilizers (ligamentous and bony anatomy of the knee), secondary stabilizers (such as the quadriceps musculature), position of the tibial tubercle, and the overall coronal and rotational alignment of the lower extremity. Understanding the anatomy is crucial to evaluate and treat patients with patellofemoral instability. The medial patellofemoral ligament (MPFL) and medial patellomeniscal ligament (MPML) both play important roles as primary stabilizers of the patella. The MPFL (Fig. 1) is a purely ligamentous structure in layer II of the medial side of the knee 4 with a strength of approximately 206 N. 5 It originates on the medial femoral condyle and attaches to the proximal medial aspect of the patella and minimizes the lateral pull on the patella from the quadriceps mechanism. Although wispy in some and once thought to be present in only 35% of knees, 6 recent research has shown its presence in all knees 7-13 with an average length of 53 mm to 65 mm The MPFL originates from a small saddle between the adductor tubercle and the medial epicondyle. The origin of the MPFL was first shown by Nomura and colleagues 14 to be distal to the adductor tubercle and posterosuperior to the medial epicondyle. Recent work by others 10,11 has supported this. An elegant anatomical study by LaPrade has shown the femoral attachment of the MPFL Alaia MJ, Cohn RM, Strauss EJ. Patellar instability. Bull Hosp Jt Dis. 2014;72(1):6-17.

2 Bulletin of the Hospital for Joint Diseases 2014;72(1): Figure 1 The medial patellofemoral ligament begins on the medial aspect of the distal femur and inserts on the superomedial aspect of the patella. Note its relationship with the fibers of the vastus medialis obliqus, directly superior. (Reproduce with permission from: LaPrade RF, Engelbretsen AH, Ly TV, et al. The anatomy of the medial part of the knee. J Bone Joint Surg Am Sep;89(9): ) Figure 2 Soft tissue insertion relationships about the medial femoral condyle. The femoral attachment of the MPFL is approximately 10.6 mm proximal and 8.8 mm posterior to the medial epicondyle and 1.9 mm anterior and 3.8 mm distal to the adductor tubercle. AT = adductor tubercle, ME = medial epicondyle, AMT = adductor magnus tendon, POL = posterior oblique ligament, GT = gastrocnemius tubercle, MGT = medial gastrocnemius tendon, and smcl = superficial medial collateral ligament. (Reproduce with permission from: LaPrade RF, Engelbretsen AH, Ly TV, et al. The anatomy of the medial part of the knee. J Bone Joint Surg Am Sep;89(9): ) to be 10.6 mm proximal and 8.8 mm posterior to the medial epicondyle and 1.9 mm anterior and 3.8 mm distal to the adductor tubercle 11 (Fig. 2). The MPML, located in layer III of the medial side of the knee, 4 originates on the inferomedial portion of the patella. It then courses inferiorly to insert on the medial meniscus anterior to the medial collateral ligament and to the tibia through the coronary ligaments 15 and has been shown to play a role in medial knee stability in full extension. 16 The trochlear groove of the distal femur is the other important primary stabilizer of the patella. It is complex in its shape, and its depth varies from proximal to distal. The trochlear geometry allows for precise articulation with the patella and provides an important primary stabilizing force against patellar translation. In full extension, the patella and trochlea are disengaged, and the primary stabilizing forces are from the medial ligaments. However, at approximately 30 of knee flexion, the distal aspect of the patella begins to articulate with the proximal end of the trochlea. The almost V shape of the trochlea in the axial view allows for the patella to remain centered and stabilized through the remainder of knee flexion. Alterations in the patellofemoral articulation, such as trochlear dysplasia or patella alta, disrupt the normal anatomic variables and subsequently predispose a patient to develop instability. There is no definitive reason why some develop trochlear dysplasia, but recent literature has provided initial insight into the possible etiology of this problem. Similar to the development of the adult hip, it is possible that a well-centered, reduced patella could stimulate the trochlea to develop anatomically. Huri and colleagues tested this hypothesis by sectioning the MPFLs in rabbits at 1 month of age. After 1 year of follow-up, controls had deeper trochlear grooves and a greater than 10 deeper trochlear groove angle. 17 The overall alignment of the lower extremity and subsequent Q angle play a significant role in stability of the patella. The Q angle refers to the angle subtended by lines drawn from the anterior superior iliac spine to the center of the patella and another line drawn from the center of the patella to the tibial tuberosity. The Q angle in females (15 to 20 ) is typically larger than that of males (8 to 15 ), which may help explain why patellofemoral instability is more common in females. Larger Q angles are also expected in patients with excessive femoral anteversion or external tibial torsion. A larger Q angle imparts a larger laterally placed vector on the patella, which becomes es-

3 8 Bulletin of the Hospital for Joint Diseases 2014;72(1):6-17 pecially important in full extension, as the tibia externally rotates and the Q angle increases. Given that the patella is completely free of confinement of the trochlear groove in full extension and lacks the posteriorly directed force of the quadriceps, the patella is at a larger risk of dislocation in an extended position. The MPFL is the most important patellar stabilizer in full extension. 12,13,15 Conlan and colleagues 15 were among the first to describe the importance of the medial ligamentous structures of the knee. In this cadaveric study, the investigators progressively sectioned parts of the medial stabilizing complex under a lateral displacement force and found the MPFL to comprise 53% of the total stabilizing force of the patella with the knee in extension. The medial patellomeniscal ligament comprised 22% of the stabilizing force. Findings from Desio and coworkers confirmed these results, 12 and it is now widely accepted that the critical structure stabilizing the patella in the first 0 to 20 of flexion is the MPFL. However, as the knee flexes and the MPFL becomes more relaxed, the trochlea becomes the primary stabilizer of the patellofemoral joint, and trochlear dysplasia has been shown to be present in up to 85% of patients with recurrent patellar instability. 18 In a 2005 biomechanical study, Senavongse and Anis 19 assessed contributions of force to lateral subluxation at varying degrees of flexion. In particular, they assessed the contributions of the trochlea, medial ligamentous structures, and the vastus medialis obliqus muscle. In full extension, the loss of the medial retinacular structures resulted in the largest reduction in force necessary to displace the patella 10 mm laterally. However, by 30 of flexion, the medial structures became the least important stabilizer. At this point, however, loss of the normal lateral condyle contour became the most significant factor contributing to lateral restraint, producing approximately a 70% decrease in force required to translate the patella 1 cm laterally. The VMO complex plays less of an important role in all angles of flexion. A complex interplay of anatomic factors contributes to pathology in patients with instability. It was recently shown in patients with acute MPFL tears that patterns of injury may be dependent on the degree of trochlear dysplasia, tibial tubercle-trochlear groove distance (TT-TG), and patellar height. 20 Also, trochlear dysplasia and TT-TG distance may be more pronounced in female patients than in male patients who sustain a patellar dislocation, helping to explain why dislocations during low-risk activities tend to be higher in women. 21 Clinical Evaluation A thorough history is paramount in the evaluation of patients with recurrent patellar instability. Patients presenting in the acute setting may describe an audible pop and have significant pain as well as hemarthrosis. Patients with recurrent instability, however, may present differently. Some may never have had a frank dislocation, and may complain of either persistent pain or subtle laxity or instability. The feeling of giving way must alert the physician to other potential sources of pathology, such as a ruptured cruciate or collateral ligament. The first incident, the number of dislocations, feelings of apprehension, as well as the profession, athletic level, and overall goals of the patient are all important information that should be gathered by the clinician. Physical examination should first focus on the overall appearance of the leg and knee. The presence of significant coronal or rotational malalignment, pathologic gait patterns, or a significant knee effusion should be noted. Range of motion of the hip and the knees should both be assessed, as well as an estimation of the Q angle. A thorough ligamentous examination should be performed to rule out other causes of instability, such as generalized hyperlaxity from connective tissue disorders. The affected knee should be palpated to assess for crepitation or sources of pain. For example, patients with a torn MPFL may have pain over the site of rupture, and patients with chondral damage or bone bruising may have tenderness at those specific sites. Lastly, a thorough examination of the patellofemoral joint must be performed. The patellar apprehension test, performed with the knee at 30 of flexion, assesses for the presence of pathologic translation. With the patient relaxed, a lateral force is applied to the medial border of the patella. Guarding or apprehension denotes a positive test. Recently, Ahmad and colleagues described an officebased variation of this examination, which the investigators termed the moving patellar apprehension test. A lateral force is applied to the patella through a full range of motion (full extension to 90, then back to full extension), and the presence of apprehension is noted. Similar to the Jobe relocation test for the shoulder, the knee is then taken through the same range of motion, this time with a medially-based force, in which the patient with recurrent instability should have relief of apprehension. The investigators found a 94.1% accuracy of this exam, with 100% sensitivity and 88.4% specificity. 22 Patellar translation may also be measured. The patella can be broken down into four quadrants in the coronal plane, and anything more than two-quadrant translation should be considered pathologic. Recently, an isolated MPML deficiency test has been described in which the examiner places a proximal-lateral force at the distal-medial insertion site of the MPML on the patella with the knee in 0 to 10 of flexion. A test is considered positive for MPML pathology when the examiner is able to uncover the medial femoral trochlea during the exam. 16 Patellar tilt may also be examined, and anything less than neutral is indicative of tight lateral structures. The grind test may also elicit pain and indicate possible intra-articular pathology. Lastly, the presence of the J sign, indicating an imbalance between medial and lateral structures, should also be determined. In full extension, the soft tissue imbalances may cause the patella to lie in a lateral position. As the knee flexes, the patella will

4 Bulletin of the Hospital for Joint Diseases 2014;72(1): Figure 3 Classification system of trochlear dysplasia as described by Dejour. (Reproduce with permission from: Dejour D, Le Coultre B. Osteotomies in patello-femoral instabilities. Sports Med Arthrosc Mar;15(1):39-46.) abruptly medialize and begin tracking in the trochlea. This motion is reminiscent of a reverse J and is indicative of patellar maltracking. Radiographic Evaluation Standard roentographic imaging remains the primary radiologic tool in the evaluation of patellar instability. Anteroposterior, lateral, and Merchant views should be obtained in all patients with suspected instability; however, the lateral view and Merchant views are typically most important. The Merchant view, 23 or an axial view of the patellofemoral joint, may evaluate the shape of the trochlea and the assessment of patellar tilt, subluxation, and the sulcus angle. This angle is subtended by two lines from the highest points on the medial and lateral condyles to the lowest point in the intercondylar sulcus. The normal angle is typically 138 ± 6, 23 and values above 145 are considered dysplastic. 18 It is important to consider that this is a measurement of the bony sulcus angle and will not reflect the structure of the articular cartilage. On the lateral view, one can assess for patella alta or trochlear dysplasia. Measurement of the height of the patella can be done in several ways; however, one of the more common techniques used is the Insall-Salvati ratio, 24 in which the distance from the distal tip of the patella to the tibial tubercle is divided by the overall length of the patella on the lateral view with the knee flexed approximately 30. A patient with a value over 1.2 is considered to have patella alta. The shape of the trochlea may also be evaluated on the perfect lateral view with the posterior femoral condyles completely superimposed. Dejour and colleagues first described the crossing sign, 18 in which the proximal trochlear floor crosses the anterior aspect of the medial and lateral femoral condyles. It has been estimated that up to 96% of patients with trochlear dysplasia have a positive crossing sign on x-ray, and a more distalized point of crossing denotes more severe deformity. On the lateral image, one can also determine the presence of a supratrochlear spur (anteriorization of the proximal trochlear floor) or the double contour sign, which signifies lateral convexity and medial facet hypoplasia. Anteriorization of the trochlear floor by the presence of a spur diminishes the relative height of the lateral femoral condyle and subsequently increases the risk of instability. A recent classification system (Fig. 3) by Dejour takes into account the presence or absence of these features and correlates them with larger amounts of dysplasia. Type A dysplasia denotes a positive crossing sign. Type B denotes a crossing sign as well as the presence of a supratrochlear spur. Type C dysplasia indicates a positive crossing sign as well as a double contour. Type D has all of the variables and signifies the most severe type of dysplasia. 25 It is very important to consider the quality of the image; however, as it has been shown that as little as 5 of rotation from a true perfect lateral may produce false positives or negatives. 26 Computed tomography may also prove useful in the radiographic evaluation of the patellofemoral joint as well as overall rotational alignment. Patellar tilt may be measured with the knee in full extension and is assessed by the angle subtended by a line tangential to the posterior femoral condyles and a line through the medial-lateral width of the patella. Angles greater than 20 signify pathologic patellar tilt. 18 Rotational alignment can be assessed using full-leg version studies. The tibial tubercle-trochlear groove distance (TT-TG), first described in the 1970s by Goutallier, 27 signifies the offset of the tibial tubercle in relation to the trochlear groove. It may be assessed on CT scan or MRI (Fig. 4). Large values signify a more lateralized tibial tubercle, and distances more than 15 mm to 20 mm 18,28,29 have been cited as pathologic and warrant attention during realignment procedures. The TT-TG distance may be measured either using contiguous images or by using two images superimposed on each other. 18,30 The TT-TG distance must be measured radiographically, as it has

5 10 Bulletin of the Hospital for Joint Diseases 2014;72(1):6-17 that the rectus femoris may play a more significant role in open-chain exercises, and that the VMO may fire with higher amplitude and in a more simultaneous manner with closed-chain exercises. 38,39 Patients opting for non-operative management should also consider changes in their lifestyle and restrict activities that may place them at a high risk for recurrent dislocation. As many patients with patellar instability have several anatomic and alignment-related factors contributing to instability, they may always be a higher risk of dislocation, regardless of a well-constructed physical therapy program. Figure 4 Measurement of the TT-TG distance using superimposed images of the trochlear groove and tibial tubercle. (Reproduce with permission from: Dejour D, Le Coultre B. Osteotomies in patellofemoral instabilities. Sports Med Arthrosc Mar;15(1):39-46.) been shown that the clinical determination of this value is unreliable. 31 MRI is also useful in the evaluation of the bony and soft tissue structures. Patients with recurrent instability will classically show either rupture or attenuation of the MFPL (up to 87% from the origin off the femoral condyle 32 ), as well as potential involvement of the MPML. Articular cartilage defects may be evaluated, as well as the presence or absence of bone bruising, most often found on the outer border of the lateral femoral condyle and the inferomedial aspect of the patella. Non-operative Management The primary goal of non-operative management in patients with PFI is to keep the patella centered in the trochlear groove throughout the range of motion. This can be done by strengthening the secondary stabilizers or by using tapes or braces. Unfortunately, there are no randomized, controlled studies comparing non-operative management to surgical management. Taping the patella has been found to increase quadriceps muscle torque, 33 increase the knee loading flexion response, 34 and activate the vastus medialis obliqus muscle earlier than the vastus lateralis during stair ascent and descent. 35,36 Taping of the gluteal muscles may also benefit the patient. Patients with weak abductors may ambulate with the leg slightly adducted and internally rotated, which would increase the Q angle and the lateral forces acting on the patella. Exercise training should focus on modification of the quadriceps and gluteal musculature. Specifically, the VMO needs to be trained to fire earlier and with more force. Some advocate that the VMO must be at least 4 times stronger than the lateralis and fire strongly early in the motion arc. 37 Also, exercises should be focused on strengthening through closed-chain exercises. Several studies support the notion Surgical Treatment As non-operative management may not have predictably good outcomes, surgical intervention is typically necessary and ranges from a vast number of procedures. As the etiology of PFI is multifactorial, there is no cookbook procedure, and it is critical to assess the contributions of proximal and distal anatomy in determining the proper reconstructive procedure. Depending on anatomical variables, patients may need proximal realignment, distal realignment, or both. Isolated Lateral Release Once routinely used in the treatment of patellar instability, it is now well known that isolated lateral release is not indicated in patients with patellar instability. 40 Rather, its use should be limited to the treatment of lateral patellar compression syndrome, characterized by patellofemoral pain and tight lateral structures, evidenced by negative patellar tilt on clinical examination. These procedures may be fraught with complications, such as pathologic medial subluxation, described by Hughston, 41 or recurrent patellar instability. Kolowich and coworkers reported that of all their failed lateral retinacular releases, 55% of those were performed in patients with a preoperative diagnosis of instability. Patients in the failed group required, on average, 2.7 more operative procedures. They recommended performing an isolated release only in patients with negative patellar tilt, less than or equal to two quadrant patellar glide, and no anatomic factors that would predispose to instability. 42 However, the use of a judicious lateral release in combination with other proximal or distal realignment procedures may be performed at the discretion of the surgeon, and great care should be taken to avoid releasing past the proximal pole of the patella in order to avoid detachment of the vastus lateralis 43 and predispose the patient to iatrogenic medial instability. MPFL Reconstruction Reconstruction of the medial patellofemoral ligament has been a well-documented method of restoring the proximal soft tissue alignment of the patellofemoral joint in cases of recurrent instability. Several graft choices have been described, including autograft (hamstring, iliotibial band), allograft, and synthetics (polyester tape) Reconstruction can be performed as a single technique or in combination

6 Bulletin of the Hospital for Joint Diseases 2014;72(1): with other procedures (lateral releases, distal realignment) as seen fit. Multiple techniques have been described, including use of suture anchors, 46 two parallel 47 or diverging patellar drill holes, 49 as well as blind tunnel fixation with biotenodesis screws. 5 Mountney recently compared four different methods of fixation and found MPFL reconstructions using a femoral through-tunnel technique and a biotenodesis screw was biomechanically superior to suture anchor fixation or blind-tunnel fixation using biotenodesis screws. 5 Some have advocated for primary repair as a possible procedure in patients with recurrent instability; however, higher rates of recurrence can be expected. 50 Results of MPFL reconstruction are typically successful, as few patients experience recurrent instability. Drez and colleagues reported an 87% satisfaction rate and no episodes of re-dislocation in their cohort at a 31.5 month follow-up after MPFL reconstruction using autograft. 46 In a series of 24 knees, Nomura and colleagues 48 reported only two episodes of recurrent instability and 88% good to excellent outcomes in the Crosby and Insall scores at 12 years of follow-up, and the Kujala score increased by 31 points on average. Twenty-one of 22 patients had evidence of patellofemoral chondromalacia; however, only two experienced a worsening of patellofemoral osteoarthritis. Fifty-eight percent of their patient cohort underwent a concomitant lateral release. In a recent study in patients with multiple dislocations and no predisposing anatomic factors, Ronga and colleagues found a redislocation rate of 10.7% and significant increases in Kujala scores. 47 Panni and associates found a 64% return to sport and found poorer results in those patients with pre-existing Outerbridge type IV chondral changes. 49 It is important to understand that many patients with patellofemoral instability will have concurrent patellofemoral chondral pathology. Multiple technical points are important to address in those undergoing MPFL reconstruction. Grafts can be malpositioned or overtensioned, which can lead to persistent pain and subsequent failure. Elias, 54 in a computational knee model, compared anatomic reconstruction with short or proximal grafts. Anatomic reconstruction had similar properties to the native patellofemoral joint; however, proximal or short grafts increased medial contact pressures at low levels of flexion, and when combined, these errors caused a peak medial pressure of more than 50% at low flexion angles. Beck found that grafts tensioned at 2 N normalized patellar translation and medial contact pressures, while overtensioning to 10 N or 40 N restricted motion and increased peak medial facet pressures during the flexion cycle. 55 Bollier and Fulkerson 53 reported on five cases of failure in which the femoral attachment was either too anterior or too proximal, with a result of either medial tilt or subluxation, medial facet overload, or recurrent instability (secondary to repetitive overload on a tight ligament). The ideal point of femoral fixation is located at or in very close proximity to Schottle s point 58 (Fig. 5), such that the graft at the insertion is as close to isometric as possible. Some have advocated Figure 5 The origin point of the MPFL, as described by Schottle. This point is located by drawing three lines. The first line, the posterior femoral cortical line, is an extension of the posterior cortex of the femur. A second line is drawn perpendicular to this line, at the point in which the femoral condyle intersects the posterior femoral cortex. A third line is drawn perpendicular to line 1, at the posterior-most aspect of the Blumensaat line. The origin of the MPFL is located just anterior to line 1 and approximately half-way between lines 2 and 3. (Reproduced with permission from: Schottle PB, Schmeling A, Rosenstiel N, Weiler A. Radiographic landmarks for femoral tunnel placement in medial patellofemoral ligament reconstruction. Am J Sports Med May;35(5):801-4.) Figure 6 Failed MPFL reconstruction in a gymnast who developed recurrent lateral instability. Note the anterior and proximal placement of the original tunnel. The proper tunnel would be more posterior and distal, as depicted by the arrow. (Reproduced with permission from: Bollier M. Fulkerson J. Cosgarea A, Tanaka M. Technical failure of medial patellofemoral ligament reconstruction. Arthroscopy Aug;27(8): )

7 12 Bulletin of the Hospital for Joint Diseases 2014;72(1):6-17 that the femoral point of fixation needs to be altered until there is a length change of less than 5 mm when going from full extension to flexion. 57 If there is too much length in flexion, then the attachment needs to be more proximal. If it is too long in extension, then the attachment needs to be distalized. In a recent MRI study, 52 it was found that only 65% of MPFL femoral insertions were placed in the correct position. There were no clinical differences; however, the patients have only been examined in the short term, and a longer follow-up in this cohort will further elucidate the complications of poorly positioned grafts (Fig. 6). The appropriate flexion at which to tension the graft is also a matter of debate. Some tension the graft in 60 of flexion 45 and believe that overtensioning at this level of flexion is difficult and should still lead to symmetric translation of the patella at 20 of flexion. Others recommend tensioning the graft at 0 to 20 of flexion, as this is the arc of motion in which the MPFL acts as the most important stabilizer of the patellofemoral joint. Other complications from the procedure may include postoperative stiffness 47,59 that may require manipulation, avulsions of the medial rim of the patella, 59 and patella fractures. 60 Thaunat suggested that medial avulsions of the patella could be secondary to weakness borne by drill holes acting as stress risers or other etiologic factors that could place stress on the reconstruction. 59 Parikh reported on five cases of postoperative patella fractures and established a classification system. Type I are transverse and through one of the drill holes and should be treated with tension band wiring. Type II are superior pole avulsions similar to a quadriceps tendon rupture and should be treated as such. Type III are the medial rim fractures described by Thaunat and may be amenable to internal fixation. 60 Distal Realignment Procedures Osteotomies Distal malalignment, evidenced by a pathologic TT-TG distance of greater than 15 mm to 20 mm, 18,28,29 should be addressed as part of the surgical management of patients with recurrent patellar instability. Osteotomies can either be strictly in the coronal plane (medialization), such as the Elmslie-Trillat procedure, 61 or an anteromedialization (AMZ) tubercle osteotomy, as popularized by Fulkerson. 62 Both have demonstrated success when utilized in the appropriate clinical situation 29,62-71 and should be part of any surgeon s armamentarium when treating those with PFI. In patients without significant patellofemoral arthritis, the Elmslie-Trillat procedure remains an acceptable means of medializing the tibial tubercle. Barber and colleagues reported a 91.4% success rate regarding instability in their cohort of 35 knees at an average follow-up of 98 months. Fulkerson and Lysholm scores greatly improved; however, there was no radiographic follow-up in this study to document any development of patellofemoral arthrosis. 65 Nakagawa and colleagues 66 had similar outcomes regarding elimination of instability; however, their group noted a higher incidence of deterioration over time. Ninety-one percent had a good to excellent outcome at a mean follow-up of 45 months; however, at the final follow-up mean of 161 months, only 64% of patients had a good to excellent outcome. Specifically, of the 27 knees that had grade 0 radiographic changes at the time of surgery, 42% had radiographic changes of grade II or worse at last follow-up. The investigators also found that those patients that had surgery within one year of their first dislocation had more favorable long-term results, suggesting that a longer period of instability (and, possibly more dislocations) could lead to articular damage that subsequently may adversely affect outcomes. Other investigators have found similar decreases in functional outcome over time with no decreases in overall stability of the patella. 67 First described in 1983, anteromedialization osteotomies (Fig. 7) offer the advantage of being able to both improve patellar tracking and offload the patellar articular cartilage by both medializing and anteriorizing the tuberosity. 62 The oblique nature of the osteotomy allows the surgeon to fine-tune the realignments in a biplanar fashion. Outcomes following these procedures have been successful. Fulkerson described a 35-month follow-up study in which 90% of patients had either good or excellent outcomes. Those with Figure 7 Anteromedialization osteotomies offer the advantage of being able to alter the amount of medialization or anteriorization, depending on the obliquity of the osteotomy. (Reproduced with permission from: Buuck DA, Fulkerson JP. Anteromedialization of the tibial tubercle: a 4- to 12-year follow-up. Oper Tech Sports Med. 2000;8:136-7.)

8 Bulletin of the Hospital for Joint Diseases 2014;72(1): Outerbridge 3 or 4 cartilaginous changes also fared well, with 75% of them having similar good to excellent outcomes. A biomechanical arm of the study showed the AMZ to equalize facet pressures past 20 of flexion and reduce lateral facet pressures in early flexion. 63 Tjoumakaris and colleagues found similar positive outcomes, a 97% return to sport rate and a 2.4% rate of recurrent dislocation in a cohort of athletes treated with AMZ and concomitant lateral release. 64 Others have documented similar success with AMZ, and cite pure instability and the lack of preoperative pain to be significant factors correlating with positive clinical results. 70,71 Given that the AMZ shifts patellar contact pressures medially and proximally, these procedures may be contraindicated in those with articular defects in these areas. Pidoriano and colleagues showed a 20% and 55% good to excellent outcome rate in those with proximally and medially or centrally located defects, respectively. On the contrary, those with distal or lateral defects had over 85% good to excellent outcomes at an average follow-up of 46.8 months. 69 Isolated AMZ in these patients, therefore, is contraindicated. However, good results have been shown recently in those that undergo concurrent AMZ with autologous chondrocyte implantation, 68 and these procedures should be considered if a distal realignment procedure is indicated. Changing the position of the tubercle has been shown to affect both kinematics and contact pressures. As little as 10 mm of overmedialization can produce as high as a 27% increase in the average contact pressures migrating to the medial facet. These investigators also found that overmedializing can cause an increase in medial compartment pressure and caution clinicians performing these osteotomies in patients with prior varus deformities or medial menisectomies. 72 Careful correction without overmedializing, though, can be expected to reduce the pressure applied to the lateral facet cartilage without causing an overload on the medial facet ; however, the pressure reductions may not be completely consistent. 75 Kinematics may also be altered following these procedures, as medializing the tuberosity can effectively rotate the tibia externally while anteromedialization may shift the tibia posteriorly compared to the malaligned position. 76 Whether or not this correlates with clinical outcome has yet to be determined. Tubercle osteotomies can be fraught with complications. Anteromedialization osteotomies have been associated with postoperative proximal tibia fractures. 77,78 Some have recommended full weightbearing only after complete radiographic and clinical healing of the osteotomy, as well as the avoidance of step cuts and use of bone graft to accelerate the healing process. 78 Patients unable to comply with weightbearing precautions should not undergo an AMZ. This does not seem to be as large a problem with the Elmslie-Trillat procedure, as this procedure requires a much smaller osteoperiosteal flap to effectively realign the extensor mechanism. Stiffness may also complicate the rehabilitation process, and the postoperative use of continuous passive motion (CPM) is recommended. Hardware prominence is quite common and may manifest with pain during daily activities such as kneeling. Patients should be counseled that up to 75% might require hardware removal. 69 As previously stated, these osteotomies may alter loading and kinematics. Trochleoplasty The term trochleoplasty refers to any procedure that changes the shape of the femoral trochlea. First described by Albee in as a lateral opening wedge osteotomy that raised the front apex of the lateral trochlea, these procedures have been indicated for patellar instability in the context of high-grade trochlear dysplasia. The original lateral opening wedge osteotomy has been abandoned by many, as even a slight anterior elevation of 6 mm has been shown to significantly elevate contact pressures and shift the mean contact pressure from the center of the patella towards the lateral facet. 80 Some still perform this procedure and feel that results can be adequate with meticulous intraoperative technique and the use of CPM postoperatively. 81 Others have since refined the procedure and many now focus on efforts to deepen the trochlear groove without significantly distorting the nature of the patellofemoral joint (Fig. 8). 82,83 However, many outside of Europe remain hesitant to perform these procedures, as they may be associated with patellofemoral joint degeneration and persistent pain. Even still, with the excellent outcomes reported by Steiner 84 in his series of patients undergoing isolated MPFL reconstruction in patients with trochlear dysplasia, some question whether or not trochlear pathology even needs to be addressed. Although trochleoplasty has been shown to recreate a more normal patellofemoral joint anatomy 85,86 and increase patellar stability, 87 outcomes after trochleoplasty have been variable. Verdonk and colleagues, in their series of 13 knees at 18 months of follow-up, reported 10 of 13 knees had a subjective rating of good or very good, with no new dislocations after the procedure. However, on an objective basis, 10 of 13 knees scored either poor or fair. The investigators noted five cases of postoperative arthrofibrosis, all of which required operative manipulation. 88 Schottle and coworkers reported a subjective improvement in 16 of 19 patients as well as reduction of patella alta and the TT-TG distance in those undergoing Bereiter trochleoplasty. Two of the three patients with a poor or fair outcome had pre-existing patellofemoral osteoarthritis, and they suggested caution before indicating trocheloplasty in those with pre-existing degenerative changes. 86 This suggestion was strengthened in another study in which all patients with postoperative pain had evidence of cartilage abnormalities preoperatively. 89 One medium-term report showed that patients with type B and D trochlear dysplasia benefitted more from trochleoplasty than those without a supratrochlear spur. 90 Long-term results have been less ideal, as a recent study showed increased patellofemoral pain in 33.4% of patients at an average follow-up of 8.3 years. Four of ten knees that

9 14 Bulletin of the Hospital for Joint Diseases 2014;72(1):6-17 sustained a recurrent dislocation, and all but one returned to sport. 83 One of the possible reasons for mixed results is the question of chondrocyte viability after these procedures, as they involve significant reshaping of the trochlea as well as fixation about the joint. Schottle and colleagues 91 reported on their results after biopsies were taken from three patients, all less than 1 year after the procedure. Histological evaluations showed normal matrixes and cell distributions, with lacunae from the underlying bone growing into the region of calcified cartilage, and confocal examinations showed almost exclusively viable chondrocytes. Although these results seem promising, more research is warranted to determine the viability of chondrocytes on a more long-term basis. Summary Recurrent patellar instability poses a considerable challenge to the clinician regarding diagnosis and treatment. Patients may have several aspects of malalignment that can be proximal, distal, or both. It is critical that the surgeon assess each problem individually and tailor his or her treatment plan to both the needs of the patient as well as the origins of the pathology. Non-operative management for recurrent instability is usually unsuccessful as patients typically have several anatomic factors predisposing them to recurrence. A lateral release is never indicated in isolation for instability; however, it may be useful in combination with other proximal or distal realignment procedures as deemed indicated by the physician. Proximal malalignment, usually in the form of a ruptured or incompetent MPFL, is best treated with reconstruction. Distal malalignment, manifested as an increased TT-TG distance, usually requires realignment in the form of a medial or anteromedial tubercle osteotomy. Trochleoplasty, although highly successful in the prevention of recurrence, may create an incongruous patellofemoral joint and predispose patients to chronic patellofemoral pain. Disclosure Statement None of the authors have a financial or proprietary interest in the subject matter or materials discussed, including, but not limited to, employment, consultancies, stock ownership, honoraria, and paid expert testimony. Figure 8 Groove-deepening trochleoplasty. An osteochondral flap is elevated from proximal to distal (1) and a high-speed burr used to deepen the trochlear groove (2). The flap is then gently pressed into the newly formed groove and fixed using two bioabsorbable sutures (3). (Reproduced with permission from: Fucentese SF, Zingg PO, Schmitt J, et al, Classification of trochlear dysplasia as predictor of clinical outcome after trochleoplasty. Knee Surg Sports Traumatol Arthrosc Oct;19(10): ) were painless before surgery remained so, and there were definitive patellofemoral osteoarthritic changes of grade 2 or worse at the last follow-up in 30% of patients. No patient References 1. Atkin DM, Fithian DC, Marangi KS, et al. Characteristics of patients with primary acute lateral patellar dislocation and their recovery within the first 6 months of injury. Am J Sports Med Jul-Aug;28(4): Fithian DC, Paxton EW, Stone ML, et al. Epidemiology and natural history of acute patellar dislocation. Am J Sports Med Jul-Aug;32(5): Hawkins RJ, Bell RH, Anisette G. Acute patellar dislocations. The natural history. Am J Sports Med Mar- Apr;14(2): Warren LF, Marshall JL. The supporting structures and layers on the medial side of the knee: an anatomical analysis. J Bone

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