Patellar instability remains a problematic entity in the athletic patient population. Most studies agree that adolescent females represent the highest risk group of patients for first-time patellofemoral dislocation. Treatment of patellar instability must be individualized, with care taken to identify both historical and anatomic risk factors. First-time instability that results in a loose chondral or osteochondral body should be addressed with both attention to the loose piece and the stabilization of the joint, most commonly in the form of medial patellofemoral ligament (MPFL) reconstruction. Treatment of recurrent instability begins with MPFL reconstruction and involves the addition of realignment procedures and/or reshaping of the dysplastic trochlea, according to the limited available literature and surgeon judgment. This chapter will focus on the recognition and treatment of patellar instability in the female athlete.
Patient Population and Predictors of Recurrence
Most studies agree that adolescent females are the highest risk group of patients for first-time patellofemoral dislocation. Other risk factors include family history, history of contralateral instability, and a number of anatomic factors.
The most predictive anatomic risk factor for patellar instability is trochlear dysplasia. Several different classification systems and strategies have been employed to describe an abnormally shaped trochlea, including the Dejour classification, sulcus angle, trochlear depth, and trochlear facet ratio. Others focus simply on the anterior height of any supratrochlear “spur,” or “boss” as predictive of recurrence. A complete understanding of the three-dimensional nature of a dysplastic trochlea can prove elusive, but many studies have concluded that trochlear dysplasia, no matter its definition, emerges as the most predictive of the risk factors for recurrence following a first-time patellofemoral dislocation. Thus many surgeons discard the precise quantification of dysplasia and classify it on a lateral X-ray or cross-sectional imaging simply as mild, moderate, or severe ( Fig. 8.1 ).
Patella alta has also been described as a risk factor for recurrent instability. This risk factor has also been measured in several ways, some of which reference either the joint line (Blackburne-Peel) or the tibia (Insall-Salvati, modified Insall-Salvati, and Caton-Deschamps). Others have focused on the relationship of the patella to the femur and/or femoral trochlea, including simple articular overlap, Biedert’s patellotrochlear index, and/or Dejour’s sagittal patellofemoral index. The latter strategy is attractive because the relationship of the patella to the trochlea seems most relevant in the measurement of patellar height. However, these measurements require magnetic resonance imaging (MRI) and can be confounded by a laterally tracking patella and difficulty identifying the precise distal location of the transition from trochlear groove to femoral condyle. In principle, patella alta indicates that a higher riding patella requires more knee flexion to engage and stabilize the patella in its trochlear groove. This places more stress on the soft tissue restraints of the knee during early flexion until bony congruity and its inherent stability take over. An axial MRI can provide clues that patella alta is present, when the patellar chondral surface is visualized without the corresponding trochlear cartilage in view ( Fig. 8.2 ).
Coronal alignment also plays a role in patellar stability. Significant genu valgum is useful to recognize during skeletal immaturity, as guided growth or hemiepiphysiodesis can be a simple intervention to resolve this problem, rather than a more invasive distal femoral osteotomy if correction is required in a skeletally mature patient. More complicated is the understanding of the path taken by the extensor mechanism as it crosses the knee. Arthroplasty surgeons often use the tibial tubercle-to-posterior cruciate ligament (PCL) distance (TT-PCL), a measure of the lateralization of the tibial tubercle relative to the PCL origin on the tibia. For patellar stability, this has not been shown to be as predictive as the tibial tubercle-to-trochlear groove (TT-TG, Fig. 8.3A ) distance, a measure of the distance from the trochlear groove nadir to the midpoint of the tibial tubercle. The difference indicates that something about the dynamic rotation of the tibia with knee flexion and extension contributes to patellar stability. More recently, attention has been paid to the location of the patellar tendon relative to the lateral trochlea ( Fig. 8.3B ). One group even found this factor to be more predictive of patellar instability than the TT-TG.
Perhaps the most poorly understood risk factor of all is femoral anteversion, in part because it is not always routinely measured in the process of evaluating an unstable patellofemoral joint. Average femoral anteversion is 15 degree, and some authors have demonstrated that even at 20 degrees, the patellofemoral joint exhibits abnormal lateral forces. Many different methods of measuring femoral anteversion have been suggested, rendering comparison of studies difficult ( Fig. 8.4 ). Literature guidance for correction of femoral anteversion in patellofemoral instability is limited, but surgeons who commonly perform derotational femoral osteotomy procedures cite a margin of error of 10–15 degrees during the procedure and therefore do not intervene unless the femoral neck is 30–35 degrees or more anteverted relative to the shaft.
Other factors such as ligamentous laxity and lower extremity motion patterns with jumping and landing have been implicated in patellar instability, but little is known precisely about how they should affect surgical management or factor into rehabilitation protocols.
Approach to the First-Time Dislocation
After a first-time patellar dislocation, the patella should be relocated as expeditiously as possible. Gentle extension of the knee, sometimes combined with pressure on the lateral patella, generally allows the patellofemoral joint to reduce. Some patients require sedation to permit relief of muscle spasm that is preventing relocation, but more commonly, the patella relocates on its own. The knee often develops a rapid hemarthrosis, resulting in quadriceps inhibition in the early postinjury period. For this reason, a brief period of immobilization is useful to prevent buckling of the knee with ambulation, and some physicians aspirate the hemarthrosis in the hopes of a faster quadriceps recovery. Prolonged immobilization should be avoided, however, as the resultant quadriceps atrophy affects the vastus medialis most quickly (due to a higher proportion of Type 1 muscle fibers), potentially worsening patellar tracking and therefore predisposing the patient to future injury. Physical therapy should focus on normalizing gait, restoring motion, and engaging the extensor mechanism without aggravating the knee pain.
Bracing an unstable patellofemoral joint can provide symptomatic relief and help control swelling by way of compression. Many different types of braces have been developed, but no studies have supported the ability of a patellofemoral brace to effectively prevent recurrent instability. This is unsurprising, given the amount of shear force experienced by the patellofemoral joint with daily activities (up to nine times body weight, depending on the activity). Likewise, a specific method of applying therapeutic tape has been developed by Jenny McConnell, a physical therapist in Australia, to help reduce symptoms of patellofemoral pain in the setting of patellar instability. When patients have a mixed picture of instability and anterior knee pain, some surgeons look to the symptomatic response to McConnell taping as predictive of any pain relief that might result from surgical stabilization of the joint.
Physical examination of an unstable patellofemoral joint begins with direct inspection. The quadriceps muscles are visually examined in standing, seated, and supine positions. Subtle atrophy can be revealed with straight leg raises or active knee extension in the sitting position. Occasionally, patients cannot actively extend due to severe maltracking or apprehension (see later discussion) but can complete a straight leg raise in a supine position. Quadriceps atrophy is important to recognize because it can be a driver of persistent symptoms, due to either buckling from muscular weakness or true recurrent subluxations of the patellofemoral joint.
Effusion is often present in the immediate aftermath of a patellar dislocation. Pathoanatomic findings such as patella alta can occasionally be identified upon inspection and palpation of the joint lines relative to the poles of the patella but are more often subtle and identified on imaging. Palpation can reveal tenderness over the medial facet of the patella and lateral aspect of the trochlea (sites of typical bony contusion) and/or at the femoral original of the MPFL between the medial epicondyle and the adductor tubercle. Crepitus can be palpated or reported subjectively and is often an indicator of chondral injury. Crepitus may be elicited during active knee range of motion during seated knee extension and/or standing squat. The compression test, during which the examiner places pressure on the patella during active knee range of motion, has been described to give some clue as to the location of a chondral injury.
Maltracking indicative of instability can be seen on physical examination by the J-sign, which is a pathologic lateral shift of the patella as the knee moves from flexion into full extension. Palpation during range of motion can also reveal deviation of the patella from the trochlear groove with more subtle instability. The pathoanatomic implications of J-sign are still under investigation but suspected contributors include high-grade trochlear dysplasia, an increased TT-TG, and patella alta. Patella alta in particular may play a role in the J-sign, as dynamic studies of patella alta have shown greater lateral displacement of the patella with the knee in increased extension.
The apprehension sign is a valuable physical examination tool in assessing the patient with patellar instability. With the knees in full extension the ‘normal’ or contralateral knee is examined first and then the ‘affected knee.’ The patella is translated medially and laterally, and the test result is considered positive if the patient develops an impending sense of instability or fear that the patella may dislocate. The test is often more sensitive for apprehension if the knee is moved from full extension through 60 degrees of flexion while placing the lateral force on the patella: the so-called ‘moving patella apprehension test.’
Patellar glide can assess for degree of instability by utilizing quadrants of lateral and medial patellar translation in a fully extended position. During tibiofemoral extension to flexion, the medial and lateral displacement is about 3 mm in each direction in a normal setting. Both sides should be examined whenever possible, as hypermobility can be detected with symmetric translation of three or more quadrants. When one knee is unaffected, lateral translation is generally found to be asymmetrically high on the injured side. In patients with a history of lateral retinacular release (LRR), medial translation can be increased and sometimes can even correlate with iatrogenic medial instability. If the patient is able to tolerate it, a dynamic examination of patellar glide can also be informative, to determine how much knee flexion is required to minimize the translation of the patella as it is stabilized within the bony confines of the trochlear groove. The dynamic patellar glide test can aid in understanding the contribution of patella alta to overall patellar instability: if the patella is still able to be subluxated laterally when the knee is flexed to 45 degrees, for instance, distalization of the tibial tubercle will likely aid in earlier patellofemoral engagement (see later discussion).
During examination of apprehension and patellar glide, the medial and lateral poles of the patella can be assessed to determine the resting or passive patellar tilt. In patients with lateral patellar tilt, the patella is gently manually everted to assess tightness of the lateral retinaculum. If the patella is fixed in a laterally tilted position, and/or excessive correction is required to reduce it to a normal position relative to the trochlea, a tight lateral retinaculum may be addressed at the time of any planned stabilization surgery (see later discussion). If the patella is easily everted to neutral on physical examination, especially in the setting of a ligamentously lax patient, the lateral retinaculum is not considered tight and should not be released or lengthened at the time of surgery.
Historically, coronal plane alignment was assessed on physical examination using the quadriceps angle (Q-angle). However, the Q-angle varies based on patient positioning, quadriceps contraction, etc. Thus cross-sectional MRI or computed tomography has become the gold standard in assessing extensor mechanism alignment using measurements such as the TT-TG distance or the TT-PCL distance, and more recently the position of the patellar tendon relative to the lateral trochlea (see later discussion).
Clinical identification of genu valgum can also be accomplished on physical examination. Supine and/or prone rotation of the hips can reveal pathologic internal rotation indicative of femoral anteversion. Observation of gait can lend clues to rotational and coronal plane alignment due to in-toeing or out-toeing, and examination of the prone thigh-foot angle can give the examiner an idea of tibial torsion, in addition to the femoral rotation maneuvers.
Imaging after an acute patellar dislocation should include radiographic evaluation to determine whether the patient has an osteochondral injury. If at all possible, standing anteroposterior and lateral views should be obtained, along with an axial view or Merchant view X-ray in early flexion. A true lateral X-ray can be difficult to obtain in patients with anatomic variabilities such as femoral anteversion or genu valgum, and radiographic technicians should be counseled to repeat an attempt if needed as the lateral X-ray is critical for assessing trochlear dysplasia and patellar height. A standing tunnel or “Salt Lake” view can help identify additional injuries in the posterior aspect of the tibiofemoral compartment. Standing bilateral hip-to-ankle radiographs can quantify coronal plane deformity, and in skeletally immature patients, they can give an idea of a leg length discrepancy at baseline, which can be useful for later comparison if surgery is undertaken around open physes. Skeletal maturity can be quantified using the traditional left-hand “bone age” X-ray and/or the recently produced atlas of physeal growth about the knee as viewed on MRI.
Most osseous abnormalities following patellar dislocation are in the form of avulsion fractures of the medial patella. Less commonly, radiography and/or MRI demonstrate an osteochondral fracture. The rate of chondral and osteochondral injury has been shown to be 70% in first-time dislocations, so routine MRI evaluation is recommended following an instability event.
Surgical Management of Patellofemoral Instability
If a loose body or osteochondral fracture is discovered in the workup of patellofemoral instability, surgical intervention is indicated to either remove a small fragment or repair a larger one. A consensus statement from the International Patellofemoral Study Group indicates that concomitant stabilization of the patellofemoral joint should be undertaken with any surgery to address a loose chondral or osteochondral fragment in the high-risk, young patients with first-time patellar dislocation (Liu et al. AJSM 2018). This is supported by a clinical study of patients treated for osteochondral injury, who showed a recurrence rate of 60% when the fragment was addressed without stabilization of the joint. Fig. 8.5 shows MRI images of a 15-year-old patient who dislocated and injured LFC, which was fixed without stabilization of the joint. After 8 months, the dislocation recurred and injured the osteochondral surface of the patella.