Radiographic Validation of a Non-invasive Method of Measuring Hip Joint Reaction Force using a Cartilage Relaxation Technique

OBJECTIVES: Joint reaction forces (JRF) and contact pressures are classically measured using destructive techniques which require dissection and interposition of materials into the joint, fundamentally altering the normal joint mechanics. Without an alternate method available, modern hip JRF biomechanical studies involve this stripping of periarticular tissues, potentially sacrificing accurate measurements. As hip arthroscopy is increasingly employed for minimally invasive treatment of hip pathology, there may be a need for minimally invasive biomechanical measurement techniques that reflect JRF more accurately. This study’s objectives were to demonstrate that a non-invasive, non-destructive technique for measuring JRF was feasible in the hip and to validate this concept using simultaneous radiographic imaging. METHODS: Twenty fresh frozen male human cadaver hemipelves were instrumented with a custom-made retrograde intramedullary nail-plate construct in the femur and an iliac-crest locking plate. These otherwise fully-intact specimens were rigidly mounted for tensile testing (Instron Model 1122). With the hip in the neutral position, the joint was distracted 5mm along the axis of the femoral shaft at a rate of 0.4mm/s, while simultaneously measuring the force required for distraction. Force-displacement curves generated allowed the best-fit polynomial to be found with resulting revelation of the native state JRF. Next, the joint capsule was vented under fluoroscopic control using an 18 gauge spinal needle and distraction was repeated from the native state with fluoroscopic images captured at 0.5mm increments. All testing was repeated three times, and two specimens were excluded for severe osteoarthritis (<2mm joint-space). RESULTS: Force-displacement curves demonstrated an initially steep but decreasing (concave-down) slope, followed by an inflection point, a linear region and then a relatively non-linear increasing (concave-up) slope with further distraction. High reproducibility between repeated measurements was found within specimens and the average native resting JRF was 115.1 N (n=18). Capsular venting decreased JRF by 16.3 N (mean post-venting=98.8 N). Under fluoroscopy, air arthrograms confirmed cartilage relaxation consistently at the measured force-distraction infection point. CONCLUSION: This study describes and validates a reproducible method of measuring hip JRF that preserves all periarticular stabilizing soft tissue structures. This is the first study in any joint to correlate force-distraction data with fluoroscopic images, confirming the method’s central premise: That JRF is equal to the measured force-distraction curve inflection point. This may be ideally suited for application in biomechanical studies of minimally invasive arthroscopic hip surgery.