The impact of the CCD-angle on range of motion and cup positioning in total hip arthroplasty.

BACKGROUND Biomechanical analysis and clinical experience reveal that offset total hip stems increase soft tissue tension and reduce the risk for dislocation in total hip arthroplasty. Most of these stems have a smaller neck-shaft-angle to increase the offset. This study investigates if changing the neck-shaft-angle has an impact on how cup and stem should be positioned with regard to range of motion. METHODS A mathematical model of a total hip arthroplasty was developed to analyze range of motion until impingement between cup and neck. Range of motion was determined for each combination of neck-shaft-angles and additional parameters like cup inclination, cup anteversion, stem antetorsion, head/neck ratio and design of the cup opening. RESULTS A maximized range of motion is achieved for neck-shaft-angles between 125 degrees and 131 degrees . Reducing the neck-shaft-angle by one degree requires reducing the cup anteversion by about 2 degrees and increasing the cup inclination by 0.45 degrees . Stems with neck-shaft-angles more than 135 degrees are not recommended when the head/neck ratio is 2.3 or less. INTERPRETATION Stems with a reduced neck-shaft-angle for an increased offset should be coupled with cups that are inclined slightly higher and less anteverted as compared to a standard stem. Precise recommendations for optimal component positioning can only be given for a specific prosthesis system regarding all parameters.

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