Abstract
Uncemented porous-coated total hip prostheses rely on osseointegration or bone ingrowth into the pores for a stable interface and long term fixation. One of the criteria for achieving this is good initial stability, with failure often being associated with migration and excessive micromotion. This has particularly been noted for long stem prostheses. To minimize micromotion and increase primary stability, a short stemmed implant ‘PROXIMA’(DePuy; Leeds, UK) with a prominent lateral flare was developed with the aim of providing a closer anatomical fit, more physiological loading and limiting bone resorption due to stress shielding. This study aims to simulate bone ingrowth and tissue differentiation around a well fixed porouscoated short stemmed implant using a mechanoregulatory algorithm and finite element analysis (FEA). Specific emphasis is made on the design of the implant and its effect on osseointegration.
An FE model of the proximal femur was generated using computer tomography (CT) scans. The PROXIMA was then implanted into the bone maintaining a high neck cut and adequate cancellous bone on the lateral side to accommodate the lateral flare and for osseointegration. A granulation tissue layer of 0.75mm was created around the implant corresponding to the thickness of the porous coating used. The mechanoregulatory hypothesis of Carter et al (J. Orthop, 1988) originally developed to explain fracture healing was used with selected modifications, most notably the addition of a quantitative module to the otherwise qualitative algorithm. The tendency of ossification in the original hypothesis was modified to simulate tissue differentiation to bone, cartilage or fibrous tissue. Normal walking and stair climbing loads were used for a specified number of cycles reflecting typical patient activity post surgery.
The majority of the tissue type predicted to be formed, simulating a month in vivo, is fibrous and indicates a weak interface proximally after this period. The stronger tissues, bone and cartilage occupy the mid-lower regions, indicating a strong interface distally. This can be explained by the unique lateral flare that provides extra stability to the distal regions of the implant, especially on the lateral side. The percentage of bone ingrown around the implant at different stages is also important and there was a significant rise from 15% after 10 cycles to about 30% after 30 cycles, simulating a month in vivo. It was also noted that initial bone formation was very high, even after a few cycles, which leads to a stronger interface early on. Fibrous tissue occupied around 45% at almost all stages and did not vary considerably.
Cartilage however, was replaced by bone as tissue differentiation occurred, reducing from about 30% after 10 cycles to 20% after 30 cycles. This further indicates the trend of tissue ossification through the regions of stronger tissues, gradually proceeding in the direction of the weaker tissues.
The unique lateral flare design and the seating of the implant entirely in the cancellous bed without any diaphyseal fixation provides contrasting results in terms of bone ingrowth around the implant. The lateral flare minimises micromotion and provides better stress distribution at the interface under the region. This accounts for a large percentage of the mid to distal regions under the flare being covered with either bone or cartilage. From the predictions of the algorithm, the significant lateral flare of the PROXIMA helps in stabilizing the implant and provides better osseointegration in the distal regions around the implant.
Correspondence should be addressed to Diane Przepiorski at ISTA, PO Box 6564, Auburn, CA 95604, USA. Phone: +1 916-454-9884; Fax: +1 916-454-9882; E-mail: ista@pacbell.net
