Summary

Study showed a simple acetabular placement plane formed by pelvic landmarks. The plane was adjusted by changing one of the landmarks to a fixed value for best representing the native acetabular orientation based on CT generated 3D pelvi

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.

Taper locking connection has been widely used in orthopedic implant devices. The long term successful clinical results indicated it is a safe and effective structural component. The common materials used are solid titanium and cobalt chromium alloys. Recently, foam metal materials showed promising results of bony in-growth characteristics and became the excellent choices for the orthopedic implants. Clinically it is desirable to taper lock the foam metal component to other structural components. To date there is no data for the foam metal being used directly in taper connection. The purpose of this study was to investigate the static locking strength of the taper junctions made of titanium foam metal comparing to that of conventional solid titanium material.

(5) 43mm long and 4mm thick sleeve were machined internally with 17mm major diameter and 3° included taper angle for each 70% porosity CP titanium foam metal and solid Ti6AL4VELI alloy materials. (10) Solid Ti6AL4VELI alloy stems were machined with OD geometry matching the ID of the sleeves. All components were inspected, cleaned and assembled to (5) pairs of each sleeve material combinations with 2224N axial compression force. Each assembled specimen was mounted on MTS Bionix test machine for torque resistance test. The angular displacement at 0.1 degree/sec was applied to the stem when sleeve was rotationally locked. The maximum torque resistance was recorded. The specimen was then re-assembled with 2224N axial compression force. Axial push out test was performed by loading at smaller end of the stem when the opposite end of sleeve was supported. The maximum push out force was recorded. Procedures were repeated for all foam metal and solid metal specimens. The taper interface surfaces were visually inspected to compare two types of sleeve materials.

The average torque resistance for foam metal and solid tapers were 20.4Nm (SD=3.68) and 21.7Nm (SD=3.72) respectively (p=0.59). The average axial locking forces were 2035.7N (SD=201.11) for foam metal taper and 1989.3N (SD= 451.84) for solid taper (p=0.839). There was no visual difference observed for tested stem outer and sleeve inner surfaces of foam metal and solid metal pairs.

This study suggested that the foam metal sleeve is capable to have comparable taper locking strength as the conventional solid taper components under dry static condition. The study indicated that the contact area does not significantly influence the friction locking. This is in agreement with the friction force definition which depends only on the coefficient of friction and normal contact force.

Ultra-high molecular weight polyethylene (UHMWPE) has been successfully used as a bearing material in total joint arthroplasty. However, longevity of these implants has been compromised by wear and fatigue damage of the polyethylene. The addition of vitamin E to the polyethylene is a process recently introduced in the market to stabilize free radicals produced during radiation crosslinking. The objective of the present study is to investigate the effect of the addition of vitamin E on the wear characteristics of UHMWPE. Sequentially cross-linked and annealed UHMWPE material (X3™, Stryker Orthopaedics, Mahwah, NJ) was used as a control.

Trident™ acetabular cups (Stryker Orthopaedics, Mahwah, NJ) with inner diameters of 36 mm and 44 mm and a wall thickness of 3.8 mm were tested on a 12 station MTS hip joint simulator. The simulator used a physiologic loading pattern with a maximum load of 2450N. The test was conducted under standard clean conditions with alpha calf fraction serum diluted to a protein concentration of 20 g/l for a total of three million cycles. All cups ran against CoCr femoral heads, and gravimetric measurements were taken every half-million cycles.

Results show that sequentially crosslinked components, size 3 6mm, had an average volume loss of 9.4 ± 2.5 mm3, while vitamin E components of the same size had an average of 16.5 ± 3.1 mm3. This represents a 75% increase for vitamin E components that is statistically significant (p = 0.039). Size 44 mm sequentially crosslinked components had an average volume loss of 6.8 ± 3.7 mm3, while vitamin E components had an average of 19.7 ± 3.2 mm3. This denotes a statistically significant increase of 192% for material with vitamin E (p = 0.011). Linear regression analysis yielded wear rates of 4.1 ± 0.9 mm3/mc and 6.1 ± 1.3 mm3/mc for size 36 mm sequentially crosslinked and vitamin E components, respectively, which represents a non-significant increase of 49% for vitamin E components. Size 44 mm sequentially crosslinked components had a wear rate of 3.8 ± 1.3mm3/mc, while vitamin E components had a wear rate of 8.1 ± 0.7 mm3/mc. This represents a statistically significant increase of 117% in wear rate for vitamin E components (p = 0.013).

The results of this testing indicate that the addition of vitamin E degrades wear performance relative to sequentially crosslinked material. Research shows that the introduction of Vitamin E affects the ability to create crosslinks during irradiation by reacting with some of the free radicals. Oral et al have shown that the crosslink density decreases when Vitamin E is blended into UHMWPE. Their research has also shown that a decrease in crosslink density causes an increase in wear rate. The results of the current testing show that the addition of vitamin E to polyethylene reduces the wear resistance of highly crosslinked polyethylene.