Instrumented testing of the patella provides safe and reliable objective data that can be used for the analysis of surgical outcomes of patella stabilizing procedures, as well as having utility in select injured states (eg) when there is a variance in the clinical vs. imaging diagnosis.
Physical examination (PE) is fundamental to patellofemoral (PF) evaluation. A major limitation is lack of quantitative measurement, resulting in an examiner-dependent qualitative exam. This work aims to overcome PF physical evaluation limitations with a more objective assessment tool.
To compare objective patellar instrumented testing to the manual PE in 2 clinical conditions: MPFL reconstruction and MPFL injury, with a non-injured opposite knee.
Patients were examined manually and with an instrumented testing machine (PORTO Knee Testing Device–patella module, Soplast-Moura, Moutinho & Morais, S.A.) which applies incremental patella load (20-105 N) in the following vectors: lateral translation, external tilt. Force applied was defined by a calculated safety factor based on published MPFL tensile strength values, and subject feedback of apprehension and/or discomfort. The test was performed within an MR scanner on 6 subjects; results were analyzed via imaging measurements. Two patients had a MR diagnosis of MPFL injury with stable manual exam to lateral patella translation, non-injured opposite knee. Four patients had MPFL surgical reconstruction with good clinical results on one side with a non-injured opposite knee. Patients were also manually examined by two orthopaedic surgeons, with manual vectors applied in the same direction. Instrumented testing results were correlated with those of manual PE and imaging measurements. The manual force applied during lateral patella translation was measured by four examiners in two subjects/each, and compared to the forces used in the instrumented device.
The manual exam among four examiners revealed an average force of 30 N (25-38). For the instrumented device: the force profile curve showed no significant changes in patellar displacement values (inter-patient) after 60-65 N. However, patient-specific ligament profile curves revealed large differences in individual ligament stiffness as well as lateral patellar translation and tilt.
In the four patients with MPFL reconstructions, force-displacement curves exhibit a similar profile compared to their opposite knee, however stronger and stiffer with higher force applications. There was greater restraint to lateral tilt in the reconstructed side in one patient compared to their opposite knee.
For the two patients with imaging evidence of MPFL rupture without PE evidence, the testing results revealed similar lateral translation to the un-injured knee, revealing normal/non-pathologic motion.
This instrumented testing device was able to safely document maximum patellar excursion tolerated by patients at a constant pace, with greater forces than the manual examination. Force-displacement ligament profile curves provides accurate objective results which can be repeated at various time intervals. This has added value in the analysis of our surgical outcomes, which can be compared to patient subjective outcome. It also has utility in select injured states when there is a variance in the clinical vs. imaging diagnosis.
The results obtained with an instrumented device may represent a new achievement in the study of PF pathology, potentially generating a classification grade of patellar motion enhancing our treatment algorithm, as well as providing firm objective measurements to our post-operative patient outcome analysis.