Abstract
The elements of my routine pre-op. planning include skin and scar assessment, the limb length (physical exam and radiographic assessments), the socket type, the stem type, and radiographic templating. Blood management is rarely an issue for primary total hips today and I generally do not recommend pre-operative autologous donation. I currently use a low molecular weight heparin for venous thromboembolic prophylaxis for most all patients. All of my patients have pre-operative medical clearance from a hospital intensivist.
A press-fit modular cementless socket is my “workhorse,” although I occasionally use supplemental fixation with spikes (low bone density) or screws (shallow or otherwise deficient hemisphere). Cemented fixation is reserved for hips with radiation necrosis. I use a dual-offset tapered cementless stem in most cases but will use a modular stem in dysplastic, post-traumatic, or severely osteoporotic femurs.
I template every case. My goals are to determine component sizes - “the part inside the bone” and improve the biomechanics of the hip – “the part outside the bone”. Sizing is relatively straight forward. For the socket, I use the teardrop and the superior 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.
Medialisation of the hip centre of rotation (COR) decreases the moment arm for body weight; increasing the femoral off-set 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 favourable alterations in biomechanics can improve clinical outcomes and reduce wear. Higher femoral off-set has been associated with greater hip abduction motion and abductor muscle strength. In two independent studies, higher femoral off-set has been associated with a significant reduction in polyethylene wear.
The traditional arthroplasty goal has been to re-create the off-set of the operated hip. In an analysis of 41 patients with one arthritic hip and one clinically and radiographically normal hip (Rolfe et al., 2006 ORS), we found that the horizontal femoral off-set of the arthritic hip was, on average, 6 mm less than that of the normal, contralateral hip. Considering this, and with medialisation of the COR, is it reasonable to make the femoral off-set a few millimeters greater than that pre-op. With modular trial components, final off-set and limb-length adjustments are made intra-operatively by assessing soft tissue tension, joint stability and range of motion.
Applying these principles in a consecutive series of 40 hips, the hip centre of rotation was medialised by 5.6 mm and the horizontal femoral off-set was increased by an average of 9.5 mm, being larger than the normal, contralateral hip by an average of 5.2 mm. This combination increased the net biomechanical advantage of the diseased hip to an average of 12.5% more than the normal, contralateral hip. The increase in femoral off-set is compensated for by medialising the COR. The average lateralisation of the proximal femur of 3.9 mm did not cause trochanteric bursitis or other pain. When the offset is right, soft tissue tension can be maintained without over-lengthening. In this series, 2.9 mm average lengthening resulted in the reconstructed limb being an average of 1.1 mm shorter than the normal side.
