Patients with different patellofemoral disorders display a distinct ligament stiffness pattern under instrumented stress testing

Objective Investigate the patellar force-displacement profile (ligament stiffness) of patellofemoral disorders. Methods Fifty-two knees from 34 consecutive patients (mean 31.6 years and 53% male) were analysed including 24 knees with patellofemoral pain (PFP), 19 with potential patellofemoral instability (PPI) and 9 with objective patellofemoral instability (OPI). Physical examination, patient-reported outcome measures (Kujala and Lysholm Scores), standard radiography and MRI or CT were performed in all patients. Instrumented stress testing (Porto Patella testing device) concomitantly with imaging (MRI or CT) was performed to calculate ligament stiffness. Results The force-displacement curves in patients with PPI and OPI displayed a similar pattern, which was different from that of the PFP group. Patients with PPI showed higher ligament stiffness (a higher force was required to displace the patella) than the patients in the OPI group. Patients with OPI had a statistically significant shallower trochlear groove and increased lateral tilt. More than half of the PPI and OPI population presented with at least one classic risk factor (patella alta, trochlear dysplasia, increased quadriceps vector, lateral tilt). In the PPI group, at least two risk factors were found in 37% of patients, whereas at least 33% of patients in the OPI group had three risk factors present. None of the patients presented with all four anatomical risk factors. Conclusion Patients presenting with patellofemoral instability (PPI and OPI) display similar ligament stiffness patterns (force-displacement curve). Patients with PFP and PPI showed higher ligament stiffness as compared with patients with OPI. Level of evidence Level V, case series.