Patellofemoral Instability: Diagnosis and Treatment

Educational Objectives

The goal of this program is to improve the management of patellofemoral instability. After hearing and assimilating this program, the clinician will be better able to:

1. Identify patients at risk for recurrent patellofemoral instability.
2. Select appropriate diagnostic measures for preoperative planning in recurrent patellofemoral instability.

Summary

Medial patellofemoral ligament (MPFL): a thin structure that tears when the patella dislocates; provides 60% of the restraining force to the patella and guides it into the trochlear groove in the initial 30° to 45° of knee flexion; ≈50% of patients have a recurrent dislocation; surgeons must determine which patients are susceptible to recurrent instability and require surgery

Nonoperative treatment: strengthening the quadriceps (especially vastus medialis oblique), hip abductors, and external rotators prevents the knee from falling into a valgus posture during athletics; patella-tracking braces and taping are helpful; immobilization is not helpful and does not reduce the probability of further dislocation; quadriceps strengthening is more important for returning to sport once functional

Surgery: indications — repeated dislocations, nonprogress with rehabilitation, or presence of risk factors for recurrent instability (ie, bilaterality, loose osteochondral fragments); vastus medialis oblique advancement and medial imbrication are not recommended

Grafts: gracilis tendon autograft is stronger than the native MPFL; semitendinosus autograft is stronger still but usually not required; allografts or autografts can be used; the graft is placed on the upper half of the patella; positioning on the femur is key; Schottle point is a small triangle at the bottom of the femur just above the posterior cortical line, a Blumensaat line, and at the intersection where the posterior condyles change in curvature from front to back; Schottle point is accurate and reproducible, but range of movement of the knee must be checked to ensure that the graft remains isometric; placement too proximally tightens the graft in flexion, causing patellar cartilage damage, graft failure, or inadequate flexion; risk for recurrent instability is low; recovery and return to sport is expected

Anatomic risk factors: recurrent instability occurs with patella alta (PA), malalignment in the coronal plane, genu valgum, and trochlear dysplasia; a tighter, stronger MPFL cannot overcome bad anatomy; the most significant risk factor is trochlear dysplasia; a combination of 2 anatomic risk factors is associated with a 50% chance of recurrent instability, and combination of 3 risk factors is associated with a likelihood of 80%

Malalignment: tibial tubercle to trochlear groove (TT-TG) distance is measured in the coronal plane using computed tomography (CT) or magnetic resonance imaging (MRI); should be measured with the perpendicular to the posterior femoral condyles and the true middle of the tibial tubercle; >20 mm is considered abnormal; the goal of correction is ≤10 mm; MRI can underestimate the TT-TG by 3 to 4 mm; measurement of the patellar tendon to the lateral trochlear ridge (PT-LTR) is more sensitive and specific for recurrent instability than TT-TG; on axial view, the amount of patellar tendon that overhangs the apex of the LTR is measured; a PT-LTR >5.5 mm is >70% sensitive and ≈90% specific for further instability; perform a tibial tubercle osteotomy or Fulkerson procedure if TT-TG is >20 mm, PT-LTR is >6 mm, and there is evidence on examination of lateral tracking and tilt; elevated PT-LTR without an elevated TT-TG can occur, especially in PA

Genu valgum: if suspected on examination, full-length standing x-rays should be taken; if valgus of the mechanical axis is >6° or the mechanical axis passes >50% into the lateral compartment, a distal femoral opening-wedge osteotomy can correct the valgus to neutral and correct the TT-TG by 7 to 10 mm; a femoral head allograft cut into a wedge can be used as a placeholder into the opening wedge when the plate is applied; in patients with >12 mo of growth remaining, a physeal tether or 2-hole plate on the opposite side can be used

Rotation: on examination, if there is more prone internal rotation than external rotation by 30°, or prone internal rotation is >90°, CT is indicated to analyze hip vs knee femoral anteversion; if anteversion is >35°, rotational osteotomy should be considered; there are implications for a lateral force factor from the quadriceps

Patella alta: the patella is wholly disengaged from the trochlear groove, and the knee must flex to a greater degree for the patella to engage in the groove for bony stability; the Insall-Salvati ratio has been replaced by the Caton-Deschamps (CD) index, in which the length of the patella to the tibia is divided by the articular length of the patella; a CD index >1.2 on MRI is considered PA; unlike the Insall-Salvati ratio, the CD index focuses on the articulation; another measure is the patellotrochlear index (also done on MRI), which measures the amount of cartilage overlap there is in extension of the trochlea vs the patella; if <25%, it is functionally PA; a short trochlea can behave like a high-riding patella, manifesting as a J-sign as the patient actively extends the knee; a jumping J-sign indicates PA with dysplastic trochlea, and this combination of anatomic risk factors is important; in extreme PA, the patella can be moved distally beyond the dysplastic part of the trochlea to overcome the dysplasia; PA is corrected by moving the tibial tubercle distally; correction ≤1 cm can be done with a feathered osteotomy and overlap over the tibial shaft; for larger corrections, consider a step cut, in which a block of bone is removed and slid down until there is bony contact; tibial tubercle osteotomies should be fixed with 3 screws for better security and to avoid cracks at the distal aspect of the shingle; the goal of PA correction is a CD ratio of 1.1

Trochlear dysplasia and trochleoplasty: on lateral x-ray, a crossing sign is seen when Blumensaat line is extended anteriorly and followed to the anterior aspect of the knee, and crosses the apex of the lateral trochlear ridge; if it extends anterior to the femoral shaft, it is a supratrochlear spur; convex shape of the proximal trochlea on CT or MRI is associated with the jumping J-sign; the height of the spur is quantified on sagittal MRI; trochleoplasty — indicated in Dejour type B or D trochlea with convex shape, spur height >7 mm, and jumping J-sign, especially for PA, short trochlea, or revision procedures; the speaker performs deepening trochleoplasty using the Dejour approach with an osteochondral shell, in which the spur is removed with an osteotome that starts flush with the anterior femoral cortex and removes a wedge of bone around the articular margin; the osteochondral shell is undermined with a high-speed burr and a cutting guide, leaving a 5-mm thick shell; a crease is cut in the center for the new groove; the shell is malleable and can be depressed in the center and reshaped with a small incision on the lateral side, allowing it to hold the 2 shingles in their new position with 2-0 polyglactin 910 (VICRYL) sutures and knotless suture anchors to compress it into the desired shape; the base of the trochlear groove then sits flush with the anterior femoral cortex on lateral view; benefits — high patient satisfaction, no radiographic progression to arthritis, and high return to work and sport; early postoperative mobility is required to prevent stiffness

Other procedures: lateral release alone cannot treat patellar instability as it only balances the patella; lateral Z-lengthening is preferable

Readings

Carstensen SE, Feeley SM, Burrus MT, et al. Sulcus deepening trochleoplasty and medial patellofemoral ligament reconstruction for patellofemoral instability: a 2-year study. Arthroscopy 2020; 36:2237-2245. doi: 10.1016/j.arthro.2020.04.017; Duchman KR, Bollier MJ. The role of medial patellofemoral ligament repair and imbrication. Am J Orthop. 2017; 46:87-91; Huntington LS, Webster KE, Devitt BM, et al. Factors associated with an increased risk of recurrence after a first-time patellar dislocation: a systematic review and meta-analysis. Am J Sports Med. 2020; 48:2552-2562. doi: 10.1177/0363546519888467; Ménétrey J, Putman S, Gard S. Return to sport after patellar dislocation or following surgery for patellofemoral instability. Knee Surg Sports Traumatol Arthrosc. 2014;22(10):2320-2326. doi:10.1007/s00167-014-3172-5; Mistovich RJ, Urwin JW, Fabricant PD, et al. Patellar tendon-lateral trochlear ridge distance: a novel measurement of patellofemoral instability. Am J Sports Med. 2018; 46:3400-3406. doi: 10.1177/0363546518809982.

Disclosures

For this program, the following relevant financial relationships were disclosed and mitigated to ensure that no commercial bias has been inserted into this content: Dr. Diduch reported nothing to disclose. The planning committee reported nothing to disclose.

Acknowledgements

Dr. Diduch was recorded exclusively for Audio Digest using virtual teleconference software. Audio Digest thanks the speakers for their cooperation in the production of this program.

CME/CE INFO

Accreditation:

The Audio- Digest Foundation is accredited by the Accreditation Council for Continuing Medical Education to provide continuing medical education for physicians.

The Audio- Digest Foundation designates this enduring material for a maximum of 0 AMA PRA Category 1 Credits™. Physicians should claim only the credit commensurate with the extent of their participation in the activity.

Audio Digest Foundation is accredited as a provider of continuing nursing education by the American Nurses Credentialing Center's (ANCC's) Commission on Accreditation. Audio Digest Foundation designates this activity for 0 CE contact hours.

Lecture ID:

OR450101

Expiration:

This CME course qualifies for AMA PRA Category 1 Credits™ for 3 years from the date of publication.

Instructions:

To earn CME/CE credit for this course, you must complete all the following components in the order recommended: (1) Review introductory course content, including Educational Objectives and Faculty/Planner Disclosures; (2) Listen to the audio program and review accompanying learning materials; (3) Complete posttest (only after completing Step 2) and earn a passing score of at least 80%. Taking the course Pretest and completing the Evaluation Survey are strongly recommended (but not mandatory) components of completing this CME/CE course.

Estimated time to complete this CME/CE course:

Approximately 2x the length of the recorded lecture to account for time spent studying accompanying learning materials and completing tests.