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Correct femoral location is key for successful MPFL reconstruction
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In most traumatic patellar dislocations, the medial patellofemoral ligament is either torn or attenuated to the point where it is functionally incompetent. Unfortunately, as the medial patellofemoral ligament is the primary static restraint to lateral patellar translation from 0° to 30°, this injury leaves the patient susceptible to subsequent dislocations. Secondary patellar stabilizers are not substantial enough to compensate for the loss of this powerful restraint, and thus, surgery is often needed for recurrent instability.
While primary repair is an option, the procedure is technically challenging and is fraught with early and late failures. Repair is a good option if the patient has a primary “peel off” lesion of the patella. In these cases, the torn medial patellofemoral ligament (MPFL) can be viewed arthroscopically around the medial side of the patella and is readily seen on MRI. The ligament can be re-attached using sutures and possibly suture anchors. Primary repair on the femur can be done using suture anchors if the surgeon can be confident the MPFL is fully intact at the patella. Fluoroscopy is important to select the correct attachment point. There is considerable potential for this repair procedure to go wrong.
Postoperative sunrise radiograph demonstrates the short, oblique patellar tunnels, which are 3 mm in diameter. Notice that most of the patella is not violated by the drill holes, thus decreasing the fracture risk.
Images: Diduch DR and colleagues
Imbrication of the MPFL is also not recommended since most MPFL tears are off of the femoral origin. Therefore, as the ligament is surgically tightened in the middle, it remains unanchored to the femur and consequently provides no significant patellar stabilization. Thus, an MPFL reconstruction, rather than repair, should be the preferred technique for most patients. In addition, it is important to rule out anatomic factors that are associated with patellar instability and thus place the reconstruction at risk if not addressed concurrently. These include patella alta, trochlear dysplasia, and malalignment as measured by increased tibial tubercle-trochlear groove distance.
After deciding to reconstruct the MPFL, the surgeon must first decide on the graft choice. The mean tensile strength of the MPFL is not high (approximately 208 Newtons), thus nearly all graft options will be stronger than the native MPFL. At our institution, the gracilis tendon is the most commonly used graft as it has appropriate strength, length (around 20 cm to 25 cm are needed in most knees), and diameter.
For an isolated MPFL reconstruction, we use a single utilitarian longitudinal incision from the pes anserine insertion to halfway between the MPFL femoral origin and the medial border of the patella. This incision allows for both graft harvest and placement. Alternatively, three separate incisions can be made over the patella, medial femoral epicondyle, and pes insertion.
Intraoperative localization of Schottle’s point is the key to this procedure, and a perfect lateral radiograph is the only way to obtain this. The red line is drawn down the posterior femoral cortex. The blue line marks the transition of the curve of the posterior femoral condyle and is perpendicular to the red line. The green line is at the posterior aspect of Blumensaat’s line and is also perpendicular to the red line.
Patellar attachment
The native insertion of the MPFL on the patella is along the proximal half to third of its medial side and is relatively broad at 17 mm. We have found that obtaining an anatomic attachment site on the patella is not as critical as on the femur. It is not clear whether there are advantages in securing the graft in one or two locations on the patella. Since it is 17-mm wide, we use a double attachment to increase the graft footprint. There are various ways to attach the graft: suture anchors, a graft looped through short oblique tunnels, transpatellar sutures, suspensory techniques, or tenodesis screws. None of these techniques have demonstrated superiority in the literature, and at our institution, we use suture anchors or a looped graft through short oblique tunnels. Transverse patellar drill holes have fallen out of favor due to the stress riser and thus increased fracture risk.
Our preferred method is to use two short oblique tunnels in the patella 3 mm in diameter (Figure 1). Mineral oil lubrication can aid with graft passage as it is looped over the anterior patella. We have only had one postoperative fracture from a patient falling directly onto his knee. Importantly, this technique saves money as there is no associated implant cost. After the graft is passed through the patella, it is tunneled between the layers of the synovium and the medial retinaculum.
Femoral location is crucial
After localizing Schottle’s point, the beath pin is advanced into the medial femoral condyle and the two graft tails are wrapped around the pin. The knee is taken through a full range of motion to check isometry prior to fixation.
The femoral location is the key. Where you secure the graft on the femur makes all the difference in the outcome, so be anatomic and be precise. The anatomic origin is found between the medial epicondyle and the adductor tubercle and is roughly 15-mm wide. We use the fluoroscopic technique popularized by Schottle and colleagues to find the femoral origin of the MPFL. It is important to obtain a perfect lateral of the distal femur with the femoral condyles overlapping. Even minor variations from a perfect lateral radiograph will result in significant errors in the tunnel placement. The first reference for Schottle’s point is a line continued from the posterior femoral cortex. The second reference uses two lines which are perpendicular to the first one: one at the posterior aspect of Blumensaat’s line, and the second at the transition of the curve of the posterior femoral condyle. Schottle’s point rests 2 mm anterior to the posterior cortical line between the two perpendicular lines (Figure 2). Take time to find this location.
Once the tunnel location has been identified with a beath pin, we loop the graft around the pin to check its isometry throughout the entire knee range of motion (Figure 3). Closely observe how the graft behaves. Cadaveric studies have taught us that the MPFL is not truly isometric, but it is mostly isometric between 0° and 100°. The MPFL should be slightly looser in deep flexion where the patella is thoroughly stabilized by the trochlear groove. Especially be wary if the tension increases as the knee goes into flexion. A graft tight in flexion will cause a loss of knee flexion, increased joint forces resulting in chondrosis, and possibly medial patellar subluxation, all resulting in poor outcomes.
The cam shape of the distal femur directly impacts the MPFL tension though an arc of motion. A femoral attachment misplaced too proximal will result in excess graft tension in deep flexion as the graft tries to stretch across the long anterior to posterior dimension of the femoral condyles. We call this error “high and tight” to refer to its high position (proximal) and subsequent abnormal tightness of the graft in flexion. Conversely, a distally misplaced femoral attachment moves the graft closer to the patella in deep flexion, which we term, “low and loose.” After localizing Schottle’s point, the beath pin is advanced into the medial femoral condyle and the two graft tails are wrapped around the pin. The knee is taken through a full range of motion to check isometry prior to fixation.
After the whipstitch is pulled through the far cortex, the graft is fixed in 30° to 45° of knee flexion with 2 Newtons (approximately 0.5 pounds) of tension. We use an interference screw to secure the graft.
After appropriate graft isometry has been obtained, the next step is fixation. With the gracilis tendon, we typically drill a 7-mm tunnel with a cannulated drill bit over the beath pin. Advance the beath pin through the far cortex and lateral soft tissues, thread the sutures through the eyelet, and pull the pin out of the femur and the graft into the tunnel. Pull only 2 Newtons or 0.5 pounds of tension on the graft, as overtensioning can be deleterious to patellar tracking. The patella should be well-seated in the trochlea at 30° of flexion. The easiest mistake is to overtension the graft, and, due to the vectors acting upon it, this can create very high contact pressures along the articulation of the medial patellar facet and the medial wall of the trochlea. This can result in a poor outcome with pain and chondrosis. We typically fix the graft with a 7-mm absorbable interference screw (Figure 4).
For rehabilitation, typically patients do not need to be braced or have restricted weight-bearing unless concomitant bony procedures are performed. Patients can usually wean off crutches between 2 weeks to 4 weeks as their gait normalizes.
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For more information:
M. Tyrrell Burrus, MD; Evan J. Conte, MD; and David R. Diduch, MD, can be reached at the Department of Orthopaedic Surgery, University of Virginia Health System, P.O. Box 800159 HSC, Charlottesville, VA 22908; Burrus’s email: mtb3u@hscmail.mcc.virginia.edu; Conte’s email: ejc8b@hscmail.mcc.virginia.edu; Diduch’s email: drd5c@virginia.edu.
Disclosures: Burrus, Conte and Diduch have no relevant financial disclosures.