In vivo quantification of patellofemoral joint contact characteristics

This study was aimed at quantifying the in-vivo contact characteristics of the patellofemoral joint, as a function of knee alignment, joint loading and anterior cruciate ligament (ACL) integrity. Two new techniques were developed. The first (non-invasive) technique was developed for use in humans, based on an analytical model of joint contact using B-spline curve fitting and surface reconstruction methods, and geometric models to predict joint contact forces and average contact stresses. The loaded patellofemoral joint geometry was obtained using magnetic resonance imaging. The force applied during loading was measured. The maximal error in the predicted surface area and contact area, evaluated with a known analytical shape and with an impression casting of a cadaveric patellofemoral joint, was $\pm$10%, and $\pm$6%, respectively. Assessment of five subjects with unilateral ACL deficiency indicated that loss of ACL integrity was associated with a reduced contact area and reduced migration of the contact region with increasing knee flexion in some of the joints. These findings may have functional implications for the development of cartilage degeneration, including exposure to excessive stresses, increased wear or inadequate cartilage nutrition.

The second technique was developed to investigate joint contact stress distributions, based on simultaneous measurements of patellofemoral contact stresses, patellar tendon forces and patellar displacements during isometric quadriceps contractions using an animal model. Contact stresses and patellar displacements were obtained using pressure sensitive film, and tendon forces were measured with an implantable force transducer. Variations in the in-vivo joint contact characteristics due to joint loading, flexion angle and ACL integrity were assessed in three cats. A substantial portion of the joint contact area was exposed to contact stresses between 6 to 13 MPa for all specimens. ACL transection resulted in decreased contact area, increased contact stresses and increased variability in patellar displacements. Preliminary results suggested that potential risk factors are common to humans and cats, including reduced magnitude and migration of contact area with increased knee flexion. The techniques developed and the proposed risk factors could provide the bases for future studies into the relations between alteration in joint contact characteristics and cartilage degeneration.