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    Treatment Options for Patellar Instability

    Most patients with patellar instability will respond to non-operative treatment. In this article, the authors review the available surgical techniques that can be used to manage patients who fail conservative measures.

    Authors

    Yair Kissin, MD, and Daniel Shein, MD

    Introduction

    Patellar instability is a very commonly seen condition. It is also a challenge to treat.

    Although most patients do not require surgery, some will fail non-operative treatment and a decision to operate will be entertained. Decision making is based on the patient’s history and complaints, along with certain objective findings that help guide treatment. [3, 4]

    Instability must be correctly diagnosed to distinguish it from other conditions causing pain. Many patients suffer from anterior knee pain, but not all of them suffer from instability, commonly characterized by:

    • Malalignment
    • Maltracking
    • Subluxation
    • Frank and recurrent dislocations in some cases

    It is important to obtain an accurate history from the patient, especially as it relates to the initial onset of symptoms and whether and under what circumstances instability has recurred. First-time traumatic patellar dislocators with no subsequent recurrence may be treated differently than patients with certain risk factors and repeated subuxation/dislocation events.

    These risk factors include: [1, 3, 4, 7, 11, 15, 16, 20, 23]

    • Female gender
    • Ligamentous laxity
    • Patella alta
    • Flat trochlea
    • Hypoplastic lateral femoral condyle
    • Family history
    • Excessive hip anteversion
    • Excessive external tibial torsion 

    Physical Examination

    The physical examination of patients with suspected patellar instability includes evaluation of the following:

    • Q angle. Normally, the Q angle is 14° in males and 17° in females. [24, 32, 36] It will be elevated in patients with patellar instability.
    • Patellar mobility. Patellar mobility can be described by the number of quadrants of displacement medially and laterally (Figure 1). [21] A laterally tracking patella and a tight lateral retinaculum may indicate patellar instability.
    • J sign. The J sign is a visible lateral displacement of the patella with motion from flexion to extension, indicating a disengagement of the patella from the trochlea. It may indicate instability in terminal extension. [12, 26]
    • Apprehension sign. Manual lateral displacement of the patella causes a sense of fear of impending dislocation and results in a contraction of the quadriceps muscle.
    • Single leg squat (SLS). Excessive hip adduction and valgus of the knee on SLS often indicates weak medial quadriceps, hip external rotators, and core strength.

    Figure 1. Gross examination of the left knee demonstrating a laterally dislocated patella.

    Imaging

    X-rays are an important screening tool for assessing:

    • Overall alignment (varus or valgus)
    • Patellar height
    • Degree of patellar engagement on the tangential view

    The tangential view should not be taken in more than 20-30° of flexion, as this may mask the presence of lateral translation and/or tilt.

    Additionally, patella alta, as noted on the lateral x-ray, can contribute to instability with the knee extended, as the patella will not engage the trochlea in early flexion as it normally does at 15-20° of flexion. [17]

    MRI can be helpful in:

    • Measuring the tibial tubercle – trochlear groove (TT-TG) interval
    • Detecting chondral damage and underlying bone marrow edema [8]

    The normal TT-TG interval is 15-20 mm, and can help in decision making if surgery is planned. In one study, the average normal interval was 12 mm, while 83% of instability patients had an interval greater than 20 mm. [8]

    MRI must be done in full extension. Many patients may exhibit lateral tilt or translation in full extension that could normalize once the knee is flexed and may or may not be the cause of the patient’s symptoms. Patellar position on the MRI, therefore, should not be the sole determining factor for surgical intervention.

    CT scan can be done at 0°, 15°, 30°, and 45° of flexion to assess:

    • Patellar position in these multiple degrees of flexion
    • Whether the patella centralizes with the knee is flexed

    In addition, a flat trochlea and/or hypoplastic lateral femoral condyle, which may contribute to instability, may be observed on CT scan. The TT-TG interval can also be measured on the CT scan, as can bony parameters such as congruence angle and sulcus angle. [22, 29]

    Non-operative Treatment

    Most conditions causing patellar instability can be treated successfully with activity modification that eases the forces across the patellofemoral articulation. Weight loss where applicable combined with proper strengthening of the associated muscle groups (vastus medialis, hip external rotators and core muscles) reduce the stress on the patella, rendering the condition tolerable.

    Occasionally, taping the patella is a helpful adjunct, and if non-operative treatment fails, can help predict success with surgical realignment procedures. [5,9]

    Non-operative treatment is usually attempted for 3 to 6 months. If that fails, surgical options are considered.

    Operative Treatment

    Surgical options for patellar instability are generally intended to centralize the patellar tracking and occasionally to unload diseased cartilage. Realignment procedures are subdivided into proximal soft tissue procedures and distal bony procedures.

    The two can be combined to centralize a laterally tracking patella. Anteriorization of the tibial tubercle is also considered to unload cartilage defects.

    Following is an evaluation of operative treatment options that are considered when a patient needs surgery to correct patellar instability:

    Lateral retinacular release (LRR): One soft tissue procedure generally to be avoided in the face of true instability is lateral retinacular release. This often-overused procedure should be reserved in rare cases of isolated patellar tilt, without subluxation or translation of the patella laterally. LRR can even worsen instability, as it results in a greater Q angle and removes a restraint in an already unstable patella. Lateral release outcomes are unpredictable at best, even in treatment of isolated patellar tilt without translation. [2,31,35]

    Radiofrequency should be used cautiously in this procedure because it has been known to cause chondral damage, presumably from the excessive heat generate by continuous use of the probe. [14]

    Proximal realignment and medial patellofemoral ligament (MPFL) reconstruction: Proximal soft tissue procedures focus on restoring the medial tether, like a seatbelt, that provides an endpoint to lateral translation. Classically, vastus medialis obliqus (VMO) advancement was described by Insall, [19] and has stood the test of time as a very successful procedure.

    This procedure is performed through a midline incision, followed by medial parapatellar arthrotomy and open lateral release. The medial advancement involves suturing the VMO more laterally and distally using several non-absorbing sutures to replace the attachment approximately one-third to one-half way across the patella. It is critical to test the advancement for proper tracking and not to overtighten the repair.

    One significant downside to this procedure is poor cosmesis, as it is done through a sizeable incision and may result in a noticeable bulge caused by the muscle advancement.

    Recently, MPFL reconstruction has become a more anatomic, minimally invasive soft tissue procedure that restores the medial tether. Either autograft hamstring or allograft hamstring or tibialis anterior can be used.

    Initially, a diagnostic arthroscopy of the knee allows direct visualization of intra-articular pathology (Figure 2). Next, the procedure is performed through two small incisions: one at the medial border of the patella, and the other over the medial epicondyle.

    Figure 2. Pre-reconstruction arthroscopic view of the left knee demonstrating abnormal lateral patellar overhang.

    The graft is fashioned and folded over in a U shape, requiring a minimum graft length of 220 mm to sufficiently engage the femoral tunnel (Figure 3). The two free ends of the graft are fixed to the medial side of the patella through a variety of fixation techniques (Figure 4).

    Figure 3. Intraoperative preparation of semi-tendinosus allograft.

    Figure 4. Intraopertative MPFL reconstruction: the free ends of the allograft have been secured within their medial patellar tunnels.

    Drill holes that completely traverse the patella have been reported to risk postoperative patella fracture, so care must be taken when using this technique. [25] Alternatively, tenodesis screws obviate the need to drill completely across the patella, while obtaining solid fixation.

    Next, the isometric point on the femur adjacent to the medial epicondyle is identified using fluoroscopy, and is critical in achieving a proper reconstruction.This point has been radiographically studied and verified and lies approximately 1 mm anterior to the posterior cortical line of the femur, and just proximal to the posterior end of Blumenstat’s line. [27]

    A 6-mm tunnel is usually drilled across, but just short of penetrating, the lateral femoral cortex. This allows the graft to completely bottom-out in the tunnel. The two incisions are then connected by creating a tunnel between layers two and three, taking care not to be deep to layer three, as that would place the graft intra-articularly (Figures 5-6).

    Figure 5. The remainder of the allograft has been slipped underneath the medial soft tissue structures of the knee in preparation for placement across the distal femur medio-laterally.

    Figure 6. Final delivery of the allograft across the distal femur using a Beath pin.

    The folded end of the graft is passed from the patellar incision to the femoral incision and then fixed to the femoral tunnel with a bioabsorbable screw (Figure 7). This is done in 30-45° of flexion so that the patella is engaged in the trochlea and with care taken to not overtighten the graft, which can lead to pain and medial instability post-operatively.

    Figure 7. Delivery of the trans-femoral interference screw, which will secure the appropriately tensioned allograft.

    The three keys to MPFL reconstruction are:

    • Identifying the correct landmarks for ligament reattachment
    • Avoiding overtightening the graft
    • Obtaining a graft that is long enough

    Improper tunnel placement will result in improper patellar tracking. Unlike anterior cruciate ligament reconstruction, the goal here is not a tight graft, which in this situation can be disastrous for the patient (Figures 8).

    Figure 8. Post-reconstruction arthroscopic view of the left knee demonstrating restored patello-femoral joint congruity.

    It is also important to remember that lateral release is not recommended in combination with MPFL reconstruction, as this may result in excessive medialization of the patella by removing the natural lateral tether.

    We generally encourage patients to weight-bear at tolerated in a knee immobilizer or hinged knee brace locked in extension from up to 2 weeks post-operatively.

    From 2 to 6 weeks, patients may perform active and passive range of motion of the knee from 0° to 90°. At 3 weeks post-operatively, close chain quadriceps strengthening exercises are suggested, and this can progress to open chain exercises at 3 months post-operatively.

    Patients may then gradually return to non-contact sports, with a potential return to contact sports 4 to 6 months post-operatively.

    Anterior-medialization (AMZ) or tibial tubercle Fulkerson osteotomy. Addressing patellar instability distally has been described utilizing a tibial tubercle osteotomy, which has been popularized by Dr. J. Fulkerson. [13]

    This is the procedure of choice when excessive TT-TG distance is seen, and is also helpful in unloading worn cartilage. Here, the tubercle can be medialized to centralize tracking, anteriorized to unload worn cartilage, and distalized to affect patella alta.

    Anteriorization with medialization of the tibial tubercle unloads worn cartilage from the distal and lateral patella to the medial trochlea. Therefore, intact medial cartilage is essential to achieving success, otherwise more pain can likely result if significant medial chondrosis is present.

    AMZ is also an essential adjunct for patellofemoral cartilage restorative procedures to unload the treated area. Additionally, AMZ alone will not substitute for lack of a medial tether and is often combined with lateral release and/or MPFL reconstruction. [30]

    This technique is performed through an 8- to 10-cm incision extending from the lateral border of the patellar tendon to the lateral edge of the proximal tibia. The tibialis anterior muscle is stripped off the tibia and the osteotomy is planed. The plane of the osteotomy is based on the goal of the surgery.

    The osteotomy is made from anterior-medial to posterior-lateral while protecting the neurovascular bundle posteriorly; the steeper the cut, the more unloading is achieved through anteriorization, while a flat cut in the sagital plane will only achieve medialization. Anteriorization of 10 to 15 mm has been shown to decrease patellofemoral forces by 20%, [6] while excessive anteriorization can lead to skin problems.

    The degree of medialization is based on preoperative TT-TG measurement; in most cases, 10 to 15 mm is sufficient to centralize the patella. Overmedialization can result in abnormally high stress medially in the patellofemoral articulation.

    The tubercle can also be distalized to alter patellar alta, if present, but this must be done with caution as it may increase patellofemoral forces.

    Once the desired position of the osteotomy is determined, two 4.5 cortical screws are used to fixate the fragment.

    Rehabilitation generally involves protected weight-bearing with crutches and a knee immobilizer for 4 weeks to reduce the risk of post-operative fracture. At 4 to 6 weeks, closed chain quadriceps strengthening exercises can be initiated, with expectation of a full recovery by 3 to 4 months.

    The patient should delay running and more forceful activities until 8 to 12 months postoperatively to allow maximum bony healing.

    Pearls to Avoid Pitfalls

    • Non-operative treatment is the solution for most patients. Whether patients go on to surgery or not, a strengthening program will either help them avoid surgery or prepare them for the difficult post-operative rehabilitation.
    • Careful decision making regarding patient selection for surgery is critical. Aligning patient expectations with the reality of the recovery, especially if an osteotomy is planned, must be addressed pre-operatively.
    • Once surgery is planned, the decision to perform proximal vs. distal vs. combined procedures should be well thought out. A TT-TG interval over 15 to 20 mm will likely require AMZ, while lateral translation without an endpoint will likely require MPFL reconstruction.
    • With MPFL reconstruction, identify the attachment points correctly, do not overtighten the graft, and obtain a long enough graft.
    • With AMZ, taper the distal end of the osteotomy to lessen the chance of fracture, obtain bicortical fixation, medialize for tracking, and anteriorize to unload cartilage.

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