Introduction

The Caton-Deschamps index (CDI) is used to evaluate patellar height and is one potential risk factor for recurrent lateral patellar dislocation. Medial patellofemoral ligament reconstruction (MPFLR) and tibial tubercle osteotomy (TTO) are performed to stabilize the patella and improve its position as it tracks within the trochlear groove. The purpose of this study was to determine if the preoperative CDI is associated with postoperative instability, revision surgery, and patient-reported outcomes following MPFLR and TTO.

Methods

Seventy-two patients who underwent primary MPFLR and/or TTO between January 2015 and November 2019 at a single institution were assessed. CDIs were evaluated by three investigators on MRI. Patients with a CDI > 1.30 were included in the patella alta (PA) group, while those with a CDI between 0.70-1.30 served as controls. A retrospective review of clinical notes was utilized to evaluate the number of post-operative instability episodes and revisions. The International Knee Documentation Committee (IKDC) and 12-Item Short Form Health Survey (SF-12) physical and mental scores were collected at a mean of 2.7 years postoperatively.

Results

Forty-nine patients (29 males, 59.2%) underwent isolated MPFLR. Of these, 19 (38.8%) had a CDI > 1.3. The PA group was significantly more likely to experience a postoperative instability episode (36.8% versus 10.0%, p=0.023), and was more likely to return to the OR for any reason (26.3% vs. 3.6%, p=0.022). Despite this, the PA group had significantly greater postoperative IKDC (86.5 vs 72.4, p=0.035) and SF-12 physical (54.2 vs 46.5, p=0.006) scores. There was no difference between the groups in postoperative SF-12 mental scores (Control: 52.5, PA: 48.9, p=0.360). Pearson’s correlation showed a significant association between CDI and postoperative IKDC (r=0.396, p=0.022) and SF-12 physical (r=0.496, p=0.002) scores.

Twenty-four patients (3 males, 12.5%) underwent TTO, of which 7 underwent isolated TTO, and 17 combined with MPFLR. Two patients who had a distalization as part of their isolated TTO were not included in the final analysis. Five patients (22.7%) had a CDI > 1.3 and were included in the PA group. There was no difference in postoperative instability (Control: 35.2%, PA: 20.0%, p=0.519) or rate of return to OR (Control: 23.5%, PA: 0.0%, p=0.263) between the two groups. There was also no difference in postoperative IKDC (Control: 62.4, PA: 77.6, p=0.226), SF-12 physical (Control: 44.4, PA: 48.04, p=0.529) and mental (Control: 55.1, PA: 55.4, p=0.957). There was no significant association between the CDI and postoperative patient reported outcome scores.

Conclusion

Patients with pre-operative patella alta as measured by CDI had higher rates of postoperative instability and return to the OR with isolated MPFL reconstruction for patellar instability. Despite this, higher preoperative CDI was associated with greater postoperative IKDC scores and SF-12 physical scores in these patients. The preoperative CDI was not associated with any of the postoperative clinical or patient reported outcomes in patients who underwent TTO with or without MPFLR.