Abstract
Introduction
Femoral component design is a key part of hip arthroplasty performance. We have previously reported that a hip resurfacing offered functional improved performance over a long stem. However resurfacing is not popular for many reasons, so there is a growing trend towards shorter femoral stems, which have the added benefit of ease of introduction through less invasive incisions. Concern is also developing about the impact of longer stems on lifetime risk of periprosthetic fracture, which should be reduced by the use of a shorter stem. For these reasons, we wanted to know whether a shorter stem offered any functional improvement over a conventional long stem. We surmised that longer stems in hip implants might stiffen the femoral shaft, altering the mechanical properties.
Materials and Methods
From our database of over 800 patients who have been tested in the lab, we identified 95 patients with a hip replacement performed on only one side, with no other lower limb co-morbidities, and a control group:
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19 with long stem implant, age 66 ± 14 (LONG)
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40 with short stem implant, age 69 ± 9 (SHORT)
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26 with resurfacing, age 60 ± 8 (RESURF)
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43 healthy control with no history of arthroplasty, age 59 ± 10 (CONTROL)
All groups were matched for BMI and gender.
Participants were asked to walk on an instrumented treadmill. Initially a 5 minute warm up at 4 km/h, then tests at increasing speed in 0.5 km/h increments. Maximum walking speed was determined by the patients themselves, or when subjects moved from walking to running.
Ground reaction forces (GRF) were measured in 20 second intervals at each speed. Features were calculated based on the mean GRF for each trial, and on symmetry measures such as first peak force (heel strike), second peak force (toe-off), the rate at which the foot was loaded and unloaded, and step length.
Results
When measured by top walking speed, stemmed implants of either type appear slower than those which do not include the femoral shaft (resurfacing). The latter group walking speed was equal to the control group (Figure 1). When looking at the whole gait cycle at any one speed, no major differences appear in the first or second peak forces (Figure 2 – 5km/h, implanted side compared). When checked for asymmetry, resurfacing patients did not demonstrate any asymmetry between legs, while either stemmed groups demonstrated slight differences between legs in terms of force related features (Figure 3).
Discussion
We sought to show if stem length has an impact on top walking speed and asymmetry of gait. This small study contributes to that debate. We could not demonstrate any functional superiority of the short over the long stem, but the short femoral stem seems to transmit load just as well as the longer stem, allowing good load transfer at toe-off, and comparable walking speed. The results stress the advantages of non stemmed implants as published before. Our study adds to the discussion as to whether long stems are still needed in primary arthroplasty.
