Stability of small pegs for cementless implant fixation

Abstract

Most glenoid implants rely on large centrally located fixation features to avoid perforation of the glenoid vault in its peripheral regions. Upon revision of such components there may not be enough bone left for the reinsertion of an anatomical prosthesis. Multiple press-fit small pegs would allow for less bone resection and strong anchoring in the stiffer and denser peripheral subchondral bone. This study assessed the fixation characteristics, measured as the push-in (P_in ) and pull-out (P_out ) forces, and spring-back, measured as the elastic displacement immediately after insertion, for five different small press-fitted peg configurations manufactured out of UHMWPE cylinders (5 mm diameter and length). A total of 16 specimens for each configuration were tested in two types of solid bone substitute: Hard (40 PCF, 0.64 g/cm³ , worst-case scenario of P_in ) and soft (15 PCF, 0.24 g/cm³ , worst-case scenario of spring-back and P_out ). Two different diametric interference-fits were studied. Geometries with lower stiffness fins (large length to width aspect ratio) were the best performing designs in terms of primary fixation stability. They required the lowest force to fully seat, meaning they are less damaging to the bone during implantation, while providing the highest P_out /P_in ratio, indicating that when implanted they provide the strongest anchoring for the glenoid component. It is highlighted that drilling of chamfered holes could minimize spring-back displacements. These findings are relevant for the design of implants press-fitted pegs because primary fixation has been shown to be an important factor in achieving osseointegration and longevity of secondary fixation. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2765-2772, 2017.

Keywords: cementless implant fixation; glenoid; interference fit; press-fit; shoulder.

Figure 1

Left: Experimental set‐up for pull‐out tests—a peg specimen is mounted into a uniaxial Instron and pushed into a 2 × 2 × 2 cm³ clamped Sawbone block (40 or 15 PCF) with a 0.3 or 0.5 mm diametral interference fit. Top right: The five peg geometries tested. Bottom right: Test profile for the push‐in/pull‐out test which was split into three parts: push‐in of the peg in displacement control, measurement of spring‐back in force control and pull‐out in displacement control.

Figure 2

Boxplots of the push‐in force (in N) for the five different geometries tested in hard bone surrogate for interference fits of 0.3 mm (left) and 0.5 mm (right). The red lines indicate the median, the top and bottom box edges correspond to ± 2.7 SD. The black lines extend to the adjacent value, the most extreme data point that is not an outlier.

Figure 3

Boxplots of the spring‐back displacement (in mm) for the five different geometries tested in hard bone surrogate for interference fits of 0.3 mm (left) and 0.5 mm (right). The red lines indicate the median, the top, and bottom box edges correspond to ± 2.7 SD. The black lines extend to the adjacent value, the most extreme data point that is not an outlier.

Figure 4

Boxplots of the pull‐out forces ratios (Pout) for the five different geometries tested in hard and soft bone surrogates for interference fits of 0.3 mm (left) and 0.5 mm (right). The red lines indicate the median, the top, and bottom box edges correspond to ± 2.7 SD. The black lines extend to the adjacent value, the most extreme data point that is not an outlier.