Small design modifications can improve the primary stability of a fully coated tapered wedge hip stem

Abstract

Increasing the stem size during surgery is associated with a higher incidence of intraoperative periprosthetic fractures in cementless total hip arthroplasty with fully coated tapered wedge stems, especially in femurs of Dorr type A. If in contrast a stem is implanted and sufficient primary stability is not achieved, such preventing successful osseointegration due to increased micromotions, it may also fail, especially if the stem is undersized. Stem loosening or periprosthetic fractures due to stem subsidence can be the consequence. The adaptation of an established stem design to femurs of Dorr type A by design modifications, which increase the stem width proximally combined with a smaller stem tip and an overall shorter stem, might reduce the risk of distal locking of a proximally inadequately fixed stem and provide increased stability. The aim of this study was to investigate whether such a modified stem design provides improved primary stability without increasing the periprosthetic fracture risk compared to the established stem design. The established (Corail, DePuy Synthes, Warsaw, IN, US) and modified stem designs (Emphasys, DePuy Synthes, Warsaw, IN, US) were implanted in cadaveric femur pairs (n = 6 pairs) using the respective instruments. Broaching and implantation forces were recorded and the contact areas between the prepared cavity and the stem determined. Implanted stems were subjected to two different cyclic loading conditions according to ISO 7206-4 using a material testing machine (1 Hz, 600 cycles @ 80 to 800 N, 600 cycles @ 80 to 1600 N). Translational and rotational relative motions between stem and femur were recorded using digital image correlation. Broaching and implantation forces for the modified stem were up to 40% higher (p = 0.024), achieving a 23% larger contact area between stem and bone (R2 = 0.694, p = 0.039) resulting in a four times lower subsidence during loading (p = 0.028). The slight design modifications showed the desired effect in this in-vitro study resulting in a higher primary stability suggesting a reduced risk of loosening. The higher forces required during the preparation of the cavity with the new broaches and during implantation of the stem could bare an increased risk for intraoperative periprosthetic fractures, which did not occur in this study.

Fig 1

(a) The instruments for the preparation of the cavity. From left to right: canal finder rasp similar to [35], Corail compaction broach and Emphasys broach. (b) Stem designs of equal size shown from the side, the top with the corresponding impactor tips, and the back for the Corail and the Emphasys stem. (c) Parameters to quantify design modifications.

Fig 2. Position of the dynamic force cell for cavity broaching and stem impaction.

Fig 3

(a) Stem alignment according to ISO 7206–4 for (b) cyclic loading in the testing machine using a PE piston against a ceramic head for force application with (c) digital image correlation measurement to determine the relative motion between implant and femur based on markers (Ø 0.4 mm).

Fig 4. Typical force signal for a single mallet stroke with the maximum force marked.

Fig 5

Workflow to determine the contact area and the actual press-fit in the bone-implant interface based on (a) a CT-scan of the prepared bone with cavity and rubber band tag to distinguish between femurs, (b) a laser-scan of the implanted stem and (c) a laser-scan of the stem (proximal part of the stem shown in yellow, distal part in green). (d) The superimposed combination of a-c. (e) Example for the contact area (in mm, positive values above 0.5 correspond to gaps; values below 0.5 are interpreted as contact), (f) example for the actual press-fit magnitude (in mm).

Fig 6

(a) Markers for the definition of the coordinate systems for the femur and the stem. (b) Example for the translational relative motion during the two loading conditions. The end of time slot 6 corresponds to the start of the high load and the associated increase in the translational relative motion.

Fig 7

a) Forces during broaching decrease with increasing CCS ratio (Corail: R² = 0.797, p = 0.017; Emphasys: R² = 0.799, p = 0.016). (b) A similar effect but less pronounced was seen for stem implantation (Corail: R² = 0.539, p = 0.097; Emphasys: R² = 0.767, p = 0.022).

Fig 8. Stem impaction force vs. total contact area for the two stem designs.

Higher contact areas were achieved for higher impaction forces for the Emphasys stem (R² = 0.694, p = 0.039). A trend towards a correlation was seen for the Corail (R² = 0.593, p = 0.073).

Fig 9

a) Subsidence after cyclic loading for both stem designs (whiskers indicate 1.5 times the IQR), red circles highlight the largest Corail value with subsidence over 8 mm; b) Total rotation increases with subsidence for both stem designs (Corail: R² = 0.889, p = 0.005; Emphasys: R² = 0.964, p < 0.001).