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

Take-Home Points

• Use fluoroscopy, isometry, or both to double-check the femoral attachment point. Failure to do so can lead to an overtensioned or undertensioned graft caused by anisometric graft placement.
• To minimize the risk of fracture, avoid drilling transverse tunnels across the patella.
• Do not “pre-tension” the medial patellofemoral ligament graft. There should be little or no tension in the graft when the patella is centered in the groove, regardless of the angle of knee flexion.
• The angle of knee flexion during securing of the graft may be important for inaccurate femoral tunnel placement. Before final fixation of the graft, always range the knee fully to make sure full passive motion will be possible once the graft is secured.
• Understanding the anatomy of the MPFL is key before considering reconstructing: That is, fluoroscopy only suggests a “cloud” to begin assessment of the femoral attachment site and is secondary to anatomic references and check of length changes between the attachment point through range of motion. New studies demonstrate the patellar attachment is broad and extends proximally from the historical patellar attachment site to an equal distance along the distal quadriceps.

The medial patellofemoral ligament (MPFL), which is essential in preventing lateral patellar instability, becomes torn in almost 100% of dislocation events.¹ Therefore, in cases of failed nonoperative management, this important constraint should be reconstructed. Reconstruction is technically challenging, precision is needed to avoid postoperative complications, and a thorough understanding of the native MPFL anatomy is paramount.

As a thickening of the medial patellar retinaculum, the MPFL connects the medial patella to the medial femur. The femoral insertion has been described a few ways. In a cadaveric study, LaPrade and colleagues² noted that it inserts 1.9 mm anterior and 3.2 mm distal to the adductor tubercle. Radiographically, the attachment has been described by Schöttle and colleagues³ and Stephen and colleagues.⁴ These techniques are discussed in more detail later.

The MPFL is a static restraint to lateral patellar translation—it acts only as a checkrein. It functions mainly in 0° to 30° of knee flexion because once the patella engages the trochlear groove, the bony articulation guides the patella during the rest of knee flexion.⁵ Most authors agree that the native MPFL is mostly isometric, and the re-created ligament should replicate it.^6,7 Using cadaveric specimens, Steensen and colleagues⁶ found that, from 0° to 90° of knee flexion, the distance from the inferior patellar attachment to the superior femoral attachment changed only 1.1 mm.

Biomechanical studies have shown that a MPFL graft with excessive tension predisposes to postoperative abnormal patellofemoral contact pressures, which cause anterior knee pain, loss of knee flexion, and patellofemoral chondrosis.^8-10 Furthermore, an overtensioned graft can cause iatrogenic medial patellar subluxation, and an undertensioned graft may still allow for pathologic lateral patellar translation.

Anatomical Bony Insertions

Femoral Insertion

Precise localization of the proper anatomical femoral attachment of the MPFL is a crucial step in reconstruction.¹¹ Small errors in femoral location have resulted in significant loss of graft isometry, increased patellofemoral contact pressures in cadaveric models,^4,7 and increased rates of failure after both MPFL repair¹² and reconstruction.¹³ Several methods for confirming proper femoral location during surgery have been described; these methods help obviate the need for large formal dissection of the medial knee.

In a cadaveric study, Schöttle and colleagues³ described a reproducible radiographic point that precisely identifies the appropriate femoral location for MPFL graft placement. The point is located on a standard true lateral radiograph of the distal femur. First, a line is drawn extending the posterior cortex of the femur distally. Next, 2 lines are drawn perpendicular to the first: one intersecting the posterior point of the Blumensaat line, the other intersecting the transition between the posterior femoral condyle and the posterior femoral cortex³ (Figure 1).

Another radiographic method for intraoperatively identifying the anatomical MPFL femoral attachment was described by Stephen and colleagues.⁴ They used a cadaveric model to confirm radiographic findings and found that the femoral attachment point, taking the anterior-to-posterior medial femoral condyle distance to be 100%, was identified 40% from the posterior border of the medial femoral condyle, 50% from the distal border, and 60% from the anterior border. This simple “40%–50%–60%” normalizing rule for radiographically defining the femoral attachment point is another helpful intraoperative adjunct for templating the appropriate location for graft placement, but calculation in a sterile operative environment can be difficult.

Both of these techniques depend on a perfect lateral radiograph of the knee, as even minor variations in a radiograph can have a dramatic effect on the appearance of the starting point.

Palpation of bony landmarks is another method for preliminarily identifying the appropriate location for femoral pin placement. If done properly, palpation helps obviate the need for corrections when confirming location using isometry or radiography. The center of the femoral attachment of the MPFL can be located in a groove midway between the medial epicondyle and the adductor tubercle.⁴ Fujino and colleagues¹⁵ conducted a cadaveric study of 31 knees in an effort to relate osseous landmarks with the femoral attachment of the MPFL. In all knees, the adductor tubercle was a reliable osseous landmark. The anatomical MPFL attachment was 10.6 mm distal to the apex of the adductor tubercle and was consistent between knees.

Although all these options offer the best available and most reproducible methods for establishing an anatomical femoral graft insertion site, it is important to note that they are based on cadaveric specimens without recurrent patellar instability. Most knees with chronic patellar instability have associated anatomical abnormalities that are not present in nondysplastic cadaveric specimens, which may alter the relationship of osseous landmarks such as the medial epicondyle and adductor tubercle.¹⁶ In a recent study of 30 patients with chronic lateral patellar instability, Sanchis-Alfonso and colleagues¹⁶ used 3-dimensional computed tomography with these radiographic landmarks and simulated femoral graft attachment sites. They found that the methods of Schöttle and colleagues³ and Stephen and colleagues⁴ did not provide precise anatomical femoral placement. Ziegler and colleagues¹⁴ correlated the anatomical femoral location of the MPFL with the Schöttle point and found the radiographic site to be 4 mm, on average, off the anatomical location. The location of an appropriate anatomical femoral attachment should be confirmed using multiple methods, including palpation of known osseous landmarks, intraoperative fluoroscopy, and, most important, assessment of graft isometry through full range of motion (ROM).