Proximal Femoral Strain Patterns During Femoral Component Insertion and Initial Implant Stability with Two Different Stem Designs

1Sturgis, L; 1,2Lieberman JR; 1,2Meneghini RM 1University of Connecticut, Storrs, CT, 2New England Musculoskeletal Institute, Dept of Orthopaedic Surgery, University of Connecticut Health Center

Senior author: rm_meneghini@yahoo.com INTRODUCTION: In cementless total hip arthroplasty, reports of a high incidence of intraoperative periprosthetic femur fractures have been found in the literature. These fractures result from high hoop strains in the cortical bone as the surgeon impacts the stem. Femoral fractures can lead to decreased stability and implant failure, ultimately requiring additional revision surgeries. Therefore, it is important to avoid such fractures and it has yet to be determined if the two most common stem geometries ( tapered “blade” or “fit and fill” design) behave differently with regard to strain patterns or initial stability. There are three objective of this study: 1) To compare proximal strains resulting from femoral component insertion of two different stem geometries; 2) To assess and compare initial stability of the implanted stems when subjected to loading; 3) To compare fracture patterns resulting from the two different stem geometries.

METHODS: Foam cortical shell femur models (Sawbones) were used for this study. Two non-cemented press-fit stems manufactured by Stryker Orthopaedics (Mahwah, NJ) were chosen for comparison. The Accolade stem is a “blade-type” tapered wedge design, which provides mediolateral stability, while the Secur-Fit Max is a “fit-and-fill” dual-wedge design. The OmniFit EON, a cemented stem, was used for control. An initial pilot study was performed during which a trained orthopaedic surgeon impacted a total of thirteen stems (5 Accolade, 5 Secur-Fit, and 3 OmniFit EON) until considered stable. A total of twenty stems (10 Accolade and 10 Secur-Fit) were impacted using a drop tower to obtain more controlled impact (Figure 1a). The crosshead was dropped from 2.75 inches above the impaction plate approximately ten times or until less than 1.5 mm of total movement was observed over three consecutive impactions. The seating height after each impaction was recorded. To evaluate the initial stability of the stems after insertion, a multipurpose test protocol was developed for an MTS device to simulate maximum physiological loading (Figure 1b). The specimens were initially loaded axially to 200 N. The load was then increased in steps of 200 N and cycled 3 times at 0.5 Hz at each load until 800 N. Then 15 N-m of torque was applied in 2.5 N-m increments. Axial loading was then increased and cycled from 800 N up to 2000 N in the same manner as previously described. Force, displacement, torque, and angular displacement were recorded during stability testing. After loading, the twenty specimens impacted with the drop tower were then impacted to fracture by the surgeon. Fractures were documented photographically.

To collect data, six rectangular rosette strain gages were placed at the same locations on each specimen. Strain measurements were taken at 10 Hz throughout testing. Two-sample t-tests and ANOVA were used to analyze and compare maximum principal strains, change in seating height, and interface stiffness between the different implants. A p-value of 0.05 was considered statistically significant. RESULTS:

From the raw strain data, the maximum principal strains at each gage location were calculated. Peak strains for each specimen were located and then averaged for the Secur-Fit and Accolade stems so that differences in high strain regions could be noted. On the medial surface of the bone, the highest strains resulted from the Accolade stem. On the anterior and lateral surfaces, the highest strains resulted from the Secur-Fit stem.

For each specimen, the seating height of the stem was recorded at the point the stem was considered stable and then after torsional and axial loading. The change in seating height was significantly lower for the Accolade stem than the Secur-Fit stem for both surgically impacted and drop-tower impacted stems, as shown in Table 1.

Table 1. Change in seating height after loading Secur-Fit (n=5) Accolade (n=5) P-value Surgical -2.9 mm -1.3 mm 0.044 Drop Tower -1.95 mm -1.25 mm 0.021

Force vs. displacement data was analyzed during torsional and axial

loading and the linear portion of each curve was graphed to obtain the slope of the best-fit line, which represents interface stiffness. No significant difference was observed between the Accolade and Secur-Fit stems for torsional interface stiffness. However, the axial interface stiffness was significantly higher for the Accolade stems than the Secur-Fit stems, as shown in Table 2.

Table 2. Average interface stiffness during loading

Secur-Fit (n=10) Accolade (n=10)

Ave. Std. Dev. Ave. Std. Dev. P-value Axial (N/mm) 286.98 66.37 538.66 327.06 0.038 Torsional (Nm/Deg) 0.773 0.069 0.751 0.088 0.530

From observation there was a difference in the way the specimens

fractured. The Accolade stems generally produced a single medial or anterior crack, while the Secur-Fit stems produced two cracks on either side of the medial implant surface (Figure 2).

DISCUSSION:

Overall the Accolade stem seemed to produce lower strains than the Secur-Fit. The results also suggest that the Accolade stem may be axially more stable than the Secur-Fit due to higher interface stiffness and smaller change in seating height. However more research is needed to draw definitive conclusions. It is recommended that the study be repeated using human cadaveric femur specimens. Also, it would be beneficial to repeat impact testing using a higher sampling rate for strain data as resource limitations prevented it for this study.

Figure 2. Typical fracture patterns for Secur-Fit (left) and Accolade (right) Figure 1. a) Drop tower setup (left) b) MTS setup (right)

Poster No. 1002 • ORS 2011 Annual Meeting