A1190. INFLUENCE OF STEM LENGTH ON THE INSERTION PATH IN THR

Abstract

The current decade has seen a marked rise in popularity of minimally invasive hip replacement, done through a variety of surgical approaches. A specific downside to the direct anterior approach includes the significant difficulty getting a “straight shot” down the femoral canal for either straight, nonflexible reaming or broaching as with standard approaches. Improper alignment in the femoral canal can lead to sub-optimal load transfer and thus compromised fixation. The femoral broach and stem insertion path for this approach is best described as a curved one, rather than the typical straight path. Some femoral components appear to be more suitable to this technique due to their geometries. The purpose of the study was to describe the effects that the single geometric parameter, stem length, has on its insertion path into the femoral canal. Due to the potential introduction of human error associated with repetitively performing a specific motion, both a physical study and a computer generated analysis were conducted.

For the physical portion of the study, a femoral implant body of generic fit and fill geometry was designed and manufactured. The length of the stem was varied from 40 mm to 100 mm in 10 mm increments. A medium sized synthetic femur (Sawbones, Pacific Labs, Seattle, WA) was machined to match the volume of the full length stem. The insertion path constraints were defined such that the stem had to maintain the greatest allowable insertion angle while still making contact on both the medial and lateral side of the canal during translation in the X direction. To reduce the variability in applying the constraints, a single author conducted the insertion procedure for each length stem while the path was videotaped from a fixed position directly in front of the setup. The most proximal lateral point of the stem was tracked through the insertion path and the X, Y coordinates were recorded at a frequency of 2 FPS. The area under this curve, referred to as the minimum insertion area (MIA), was calculated.

For the computer generated portion of the study, a CAD model of the standard length Omnifit^® (Stryker Orthopaedics) was utilized. The stem was modified to create 5 additional models where the length was progressively shortened to 65%, 55%, 45%, 35%, and 25% of original length or 91mm, 77mm, 63mm, 49mm, and 35mm respectively. The femur was created from a solidified mesh of a computed tomography (CT) scan with the canal virtually broached for a full length stem. The models were each virtually assembled within the femoral canal with the similar constraints as the physical study. Again, the most proximal lateral point of the stem was tracked through the insertion path with the coordinates recorded and the MIA was calculated.

There was a non-linear relationship between stem length and the MIA with the rate of change decreasing as the stem length decreased. That is, the greatest decrease in MIA was between the standard length and next longest length in the computer simulation. It was noted that marked change in MIA began to subside between the 77mm and 63mm stems and continued this trend of having less influence onward through to the shorter lengths. Although the results of the physical study showed a higher variability than the computer generated portion, it does confirm the results of the computer generated study.

Minimizing the trauma associated with THR has led most of the above authors to the direct anterior approach. However, the femoral broach and stem insertion path is best described as a curved one, rather than the typical straight path used in other approaches. This curved insertion path also has benefits for other approaches since the broaches and stem can be kept away from the abductors, minimizing the potential injury to them. Shorter stem length makes this curved insertion path easier to perform. This is the first study to describe the effect that stem length has on its insertion path into the femoral canal. As expected, the physical portion of the study showed more variability than the computer generated portion. However, the physical and computer studies correlated well, with shorter stem lengths clearly allowing a more curved insertion path. The improvement tapered off in stem lengths below 63mm. This length correlates well with the other attempts at a shorter stem. This study provides quantitative data to help with shorter stem design and possible computer navigated insertion paths.