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PROSTHESIS DESIGN

Over 150,000 hip prostheses are implanted in the U.S. each year, with at least double that number world wide. Typically, joints are trouble-free for 15 years - a remarkable performance record considering the harsh biomechanical and biochemical environments of the body. However, as the expectation for this procedure continues to increase, it is being extended to younger and more active individuals where a service life of 30 years is desirable for the implant. This demands refinement of the design of these devices, requiring state-of-the-art engineering design and analysis techniques. Over the past eight years, we have developed highly sophisticated, non-linear finite element models to study fundamental aspects of prosthesis design, particularly the uncemented total hip prosthesis. These models, developed from computed tomography scans, have accurate descriptions of the three-dimensional geometry and material properties of the bone, non-linear frictional interfaces between the bone and implant, and realistic joint contact and muscle loads that simulate various activities. Our goal with this work is to understand the effects of implant design, surface treatment, bone ingrowth, and bone loss on the biomechanics of the uncemented hip arthroplasty. More recent work is also extending these analysis techniques to the study of fracture fixation for the human facial skeleton, in collaboration with the Department of Otolaryngology-Head and Neck Surgery at UCSF. The clinical motivation for all this work is threefold: improve current designs, better understand clinical results, and develop more objective guidelines for the selection of these prostheses.

To date, we have performed a number of comprehensive studies on various aspects of the biomechanical behavior of the cementless bone-implant system, ranging from fundamental studies using highly idealized geometries to more focused studies addressing surgical techniques on a specific design. Our finite element analysis work is continuing in this area as we strive to develop and refine designs before they go to animal or clinical studies. In addition, our latest area of interest is the design of lubrication into the artificial joint, which is currently just underway. The motivation for this latter work is to reduce the wear of the polyethylene component of the artificial joint, which is currently considered to be the "weak link" in the arthroplasty. As with most studies in the laboratory, our finite element analyses are complemented by experiments designed to test and confirm the model predictions.


ABOVE: Anatomically-accurate finite element model of an implanted artificial hip prosthesis.
 
 

Relevant Publications

  1. Keaveny TM and Bartel DL: Load transfer in the proximal femur with an Austin Moore press-fit prosthesis. Journal of Orthopaedic Research, 11:272Ð284, 1993.abstract
  2. Keaveny TM and Bartel DL: Effects of porous coating and collar support on early load transfer for a cementless hip prosthesis. Journal of Biomechanics, 26:1205-1216, 1993.abstract
  3. Keaveny TM and Bartel DL: Effects of porous coating, with and without collar support, on early relative motion for a cementless hip prosthesis. Journal of Biomechanics, 26:1355-1368,1993.abstract
  4. Keaveny TM and Bartel DL: Fundamental load transfer patterns for press-fit, surface-treated intramedullary fixation systems. Journal of Biomechanics, 27:1147-1159, 1994.abstract
  5. Keaveny TM and Bartel DL: Mechanical consequences of bone ingrowth in a hip prosthesis inserted without cement. Journal of Bone and Joint Surgery [Am], 77-A:911-923, 1995.abstract
  6. Konieczynski DD, Bartel DL, and Keaveny TM: The effects of implant size and friction on early stability and load transfer of anatomic cementless hip implants. Proc. ASME Summer Bioengineering Conference 1993, 556-559.
  7. Sun EA and Keaveny TM: Bone resorption can create a risk of fatigue fracture of porous coated AML prostheses. Trans. Orthopaedic Research Society 1994, 583.
  8. Keaveny TM and Jaloszynski RL: Effects of circumferential vs. medial-only porous coating for THA cementless fixation. Trans. Orthopaedic Research Society 1994, 220.
  9. Keaveny TM and Ulla IM: The effects of variations and distribution of bone ingrowth on bone stresses around a fully porous-coated hip prosthesis. ASME Int Mech Eng Congress & Expstn 1995, 279-280.
  10. Keaveny TM and Bartel DL: Letter to the Editor (Response) Journal of Bone and Joint Surgery [Am], 78-A:312-313, 1996.

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