Abstract
Introduction: Previous in vivo kinematic analyses of the hip joint have determined that femoral head separation from the medial aspect of the acetabular component occurs in metal-on-polyethylene THA. The present study analyzes subjects having either an alumina-on-alumina (AOA),alumina-on-polyethylene (AOP),metal-on-metal (MOM) or metal-on-polyethylene (MOP) THA during gait to determine if the incidence of hip joint separation varies based on articular surface material.
Methods: Forty subjects were analyzed in vivo using video fluoroscopy. Ten subjects had a AOA THA, ten an AOP THA, ten a MOM THA, and ten having a MOP THA. All THA subjects were implanted by two surgeons and were judged clinically successful (Harris hip scores > 90.0). Each subject performed normal gait on a treadmill and an abduction/adduction leg lift maneuver while under fluoroscopic surveillance. The two-dimensional (2D) fluoroscopic videos were converted into 3D using a computer automated model-fitting technique. Each implant was analyzed at varying flexion angles to assess the incidence of hip joint separation.
Results: During gait and the abduction/adduction leg lift, no separation was observed in subjects having an AOA THA or in subjects having a MOM THA. Similar to our previous studies pertaining to subjects having a THA with a polyethylene acetabular insert, all ten subjects having a MOP THA and 6/10 subjects having an AOP THA experienced hip joint separation. The maximum amount of separation was 7.4 mm for a subject having an AOP THA and 3.1 mm for a subject having a MOP THA.
Discussion: This study shows femoral head separation from the medial aspect of the acetabular component can occur in the presence of a polyethylene liner. The femoral head often remains in contact with the liner, hinging superolaterally. Potential detrimental effects resulting from hip joint separation include premature polyethylene wear, component loosening (secondary to impulse loading conditions) and hip instability. Wear may be enhanced due to creation of multidirectional wear vectors or excessive loads due to eccentric femoral head pivoting. The absence of separation observed in AOA and MOM THA designs may be related to increased wettability of these materials and tighter radial tolerances resulting in a cohesive lubrication film. This data may be of value in hip simulation studies to better duplicate wear patterns observed in retrieval analyses and assist in the understanding of the lubrication regime and wear rates in AOA and MOM designs, allowing for the synthesis of prosthetic components that minimize wear and optimize kinematics.
The abstracts were prepared by Nico Verdonschot. Correspondence should be addressed to him at Orthopaedic Research Laboratory, University Medical Centre, PO Box 9101, 6500 HB Nijmegen, The Netherlands.
