Correct Positioning of the Medial Patellofemoral Ligament: Troubleshooting in the Operating Room

The American Journal of Orthopedics. 2017 March;46(2):76-81

March 1, 2017 | The American Journal of Orthopedics

M. Tyrrell Burrus, MD ; Brian C. Werner, MD ; Jourdan M. Cancienne, MD ; David R. Diduch, MD

Medial patellofemoral ligament (MPFL) reconstruction is often required after failed nonoperative management of lateral patellar instability. It is important to properly re-create the native ligament to avoid altering patellofemoral biomechanics. Such alterations can cause knee stiffness, anterior knee pain, and patellofemoral chondrosis. Incorrect femoral location is the most common mistake that affects MPFL graft biomechanics. Authors have described multiple radiographic and anatomical landmarks that assist in determining the appropriate location, and time should be taken to accurately localize this position. Regardless of the reconstruction technique used, the knee should be taken through its full range of motion, before the MPFL graft is secured, to test the biomechanics and reduce the risk of postoperative complications. If the graft becomes too tight as the knee moves into flexion, the femoral location is too proximal and should be adjusted (“high and tight”). By contrast, if the graft becomes too loose in flexion, then the femoral location is too distal (“low and loose”). These simple rules can be used to intraoperatively troubleshoot the tunnel placement.

Patellar Insertion

The patellar attachment of the MPFL has received considerably less attention than the femoral attachment.¹¹ Anatomical studies have shown that the MPFL inserts on the superomedial half to third of the patella, in addition to a portion inserting on the undersurface of the vastus medialis.¹⁷

Re-creation of this insertion is more forgiving than the femoral attachment, and thus there are numerous acceptable options for graft configuration and fixation.^4,6,18 Two-tail grafts are thought to cover more of the native footprint.¹¹

Fixation options include suture anchors, interference screws, transpatellar sutures, suspensory techniques, and bone tunnels; none is superior over the others, according to the literature^19-22 (Figure 3). However, caution must be taken with bone tunnels, as full-width transverse tunnels can act as stress risers and may lead to patella fracture.²¹ Our preferred technique for the patellar attachment includes 2 short, parallel, oblique drill holes (3 mm in diameter) in the proximal half of the patella.

Gracilis autograft is looped through these tunnels, obviating the need for patellar fixation, decreasing implant costs, and reducing the risk of fracture by avoiding full-width transverse tunnels (Figures 4, 5A-5B).

Troubleshooting

It is essential to check graft tension through full knee ROM and observe how the graft behaves in order to prevent iatrogenic complications¹¹(Figures 6A, 6B).

Cadaveric studies have shown that the MPFL is mostly isometric between 0° and 100°, and becomes slightly looser in deep knee flexion in which the patella is stabilized by the trochlear groove.^4,6,17 These findings are attributable to the cam shape of the distal femur, which directly impacts the tension of the MPFL as the knee goes from extension into flexion. Fixing the graft on the patella first, which is less crucial in terms of position, offers the ability to loop the ends of the graft around a passing pin to assess the planned femoral fixation site. If the graft becomes tighter with knee flexion, the femoral attachment is too proximal.²³ This error is referred to as “high and tight,” meaning that a high or proximal femoral attachment produces a graft that is too tight with knee flexion. This is the worst mistake to make. If graft tension increases with increasing knee flexion, the result is loss of knee flexion or graft failure, increased contact forces resulting in patella femoral chondrosis, and possibly medial subluxation.^10,11,24 Conversely, a distally misplaced femoral attachment yields a graft that is looser in flexion, or “low and loose.” These helpful phrases describe graft behavior as the knee is brought from extension into flexion, and as such are troubleshooting aids in the operating room.²³

If the graft is secured in high degrees of knee flexion, and the femoral location is not anatomical, a different phenomenon occurs when the knee is brought back into extension. For proximal femoral tunnels, the graft loosens in knee extension and may lead to continued lateral patellar instability. On the other hand, a distal femoral tunnel may result in iatrogenic medial patellar subluxation as the graft becomes too tight in extension.

Correct Amount of Graft Tension

Overtightening the MPFL during fixation is an easy but avoidable mistake. Unlike the anterior cruciate ligament, the MPFL should not be secured while applying maximum tension. Stephen and colleagues⁷ and Beck and colleagues⁸ found that tension of only 2 N (~0.5 lb) is needed to accurately re-create the biomechanics of the native graft.

The amount of tension may inadvertently be increased by an interference screw, which tends to pull the graft into the femoral tunnel during insertion. Attention should be given to watching and palpating the graft as the screw is inserted, especially during the last few turns. Turning the screw half a turn backwards after full insertion can release this increased tension and help avoid overtensioning.

Correct Amount of Knee Flexion

This is probably the least studied aspect of MPFL reconstruction. Recommendations range from 0° to 90° of knee flexion during fixation.^7,25-30 Most recommendations are surgeon preference, or are based on a sound rationale that lacks supporting research. Tensioning in full extension has been advocated for assessing for the appropriate amount of lateral patellar translation.²⁷ Authors who endorse deeper knee flexion (60°-90°) think that, because the patella engages a deeper trochlear groove in increased flexion, the bony articulation can be used to establish graft length.^30,31

Our cadaveric study showed that lower degrees of knee flexion are safest for minimizing the effect of a malpositioned femoral tunnel.²⁶ If femoral tunnel location is not exactly anatomical, any errors are magnified (with even worse graft mechanics) the deeper in flexion the graft is fixed. Once the patella engages the trochlear groove, at about 30° of knee flexion, this can assist in establishing correct graft length. Therefore, we recommend fixation of the graft in 30° to 45° of knee flexion. Our study results also showed that, if femoral tunnel location is anatomical, the graft will be mostly isometric through knee ROM, and, therefore, amount of initial knee flexion does not affect graft behavior.

Regardless of knee flexion chosen, it is imperative to take the knee through full ROM after fixation to ensure the graft does not excessively loosen or tighten in flexion or extension.

References

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