Summary

Surgeons should be aware of this phenomenon and aim for a uniform C-arm position for every MPFL reconstruction.

Abstract

Background

Medial patellofemoral ligament (MPFL) reconstruction is an established procedure in the treatment of patellar instability. However, femoral tunnel misplacement is responsible for 38.2% of cases in revision surgery.
Different methods are published for the identification of the femoral MPFL insertion site. In clinical practice, the Schöttle technique, using C-arm can be considered the gold standard.

Purpose

The purpose of the study was to evaluate the influence of the C-arm position on the radiographic femoral MPFL positioning.
Study Design: Descriptive laboratory study.

Methods

Ten cadaveric knees were dissected, the femoral MPFL insertion site was identified and marked using 10 mm eyelets. According to possible clinical scenarios, true lateral radiographs in two different C-arm positions(ML5: 5cm from the receptor with X-ray beam from medial to lateral; LM25: 25cm from the receptor with X-ray beam from lateral to medial) were taken. At each radiograph, the eyelet position was recorded as the distance (proximal-distal and anterior-posterior) from the optimal radiographic insertion point according to Schöttle et al..
Differences were calculated using the Wilcoxon signed-rank test (2-related sample), and a p-value of less than 0.05 was considered significant.

Results

The anatomic femoral MPFL insertion in the ML5-position was located a mean of 2.3 ± 2.4 mm (range, 0.2-5.8) proximally and 4.1 +/- 6.0 mm (range, -6.2-13.6) anteriorly to the Schöttle point. This resulted in an absolute distance of 7.2 +/- 3.0 mm (range, 3.6-13.6). In the LM25-position it was located a mean of 0.6 +/- 1.8 mm (range, -3.2-5.6) distally, and 2.7 +/- 5.7 mm (range, -8.4-9.8) anteriorly, which resulted in an absolute distance of 5.5 +/- 3.1 mm (range, 1.1-9.8). The mean distance of the eylet in both positions – ML5 compared to LM25 - was 3.0 +/- 2.3 mm (range, 0-7.3) proximally and 1.4 +/- 2.3 mm (range, -2.7-4.8) anteriorly with an absolute distance was 4.1 +/- 2.1 mm (range, 2.4-8.8) in the ML5-position.
Wilcoxon signed-rank test (2-related sample) showed a significant difference between the two C-arm setups in the x-axis (proximal-distal). No significant differences could be determined when comparing the distance in the y-axis (anterior-posterior) or the absolute distance between Schöttle point and the anatomic attachment of the MPFL.

Conclusion

Intraoperative C-arm positioning in MPFL reconstruction, does affect femoral anterior-posterior tunnel positioning. Surgeons should be aware of this phenomenon and aim for a uniform C-arm position for every MPFL reconstruction.