Preop Planning for a Total Hip Arthroplasty
In the abstract from his presentation at ICJR’s Pan Pacific Orthoapedic Congress, Dr. Thomas Schmalzried discusses his goals when templating a THA case.
By Thomas P. Schmalzried, MD
I template every total hip arthroplasty case. My goals are to:
- Approximate component sizes – “the part inside the bone”
- Improve the biomechanics of the hip – “the part outside the bone”
Sizing is relatively straightforward. For the socket, I use the teardrop and the supero-lateral bony edge as landmarks for size and position. I use a Johnson’s lateral view radiograph to assess socket version and anterior osteophytes.
With a tapered stem, proximal fit on the AP radiograph is the goal, and the stem does not need to be canal filling. For the neck resection, I reference off the lesser trochanter.
Medialization of the hip center of rotation (COR) decreases the moment arm for body weight; increasing the femoral offset lengthens the lever arm for the abductor muscles. These changes in hip biomechanics have a double benefit: a reduction in required abductor forces and lower joint reaction forces.
There is accumulating clinical evidence that such favorable alterations in biomechanics can improve clinical outcomes and reduce wear. Higher femoral offset has been associated with greater hip abduction motion and abductor muscle strength.  In 2 independent studies, higher femoral offset was associated with a significant reduction in polyethylene wear. [2,3]
With modular trial components, final offset and limb-length adjustments are made intraoperatively by assessing soft tissue tension, joint stability, and range of motion. When in doubt, I take an intraoperative x-ray.
Thomas P. Schmalzried, MD, is the Medical Director of the Joint Replacement Institute at St. Vincent Medical Center, Los Angeles, California.
- McGrory BJ, Morrey BF, Cahalan TD, An KN, Cabanela ME. Effect of femoral offset on range of motion and abductor muscle strength after total hip arthroplasty. J Bone Joint Surg Br. 1995; 77:865-869.
- Schmalzried TP, Shepherd EF, Dorey FJ, et al. Wear is a function of use, not time. Clin Orthop. 2000; 381:36-46.
- Sakalkale DP, Sharkey PF, Eng K, Hozack WJ, Rothman RH. Effect of femoral component offset on polyethylene wear in total hip arthroplasty. Clin Orthop. 2001; 388:125-134.