Humeral cementless surface replacement arthroplasties of the shoulder: An experimental investigation on their initial fixation

Abstract

Cementless surface replacement arthroplasties are increasingly being used to treat arthritic humeral heads. These implants are designed to provide narrow bone resection, making a later revision easier. However, no clear evidence exists as to whether their initial fixation is sufficient for bony ingrowth. The aim of our in vitro study was to characterize the relative micromotion of two resurfacing implants with essentially different bone-facing geometries. Both systems were implanted into 10 human humeral specimens and micromotion was measured under a cyclic torque application of up to AE1.75 Nm. The mean relative rotary motion resulted in a significant difference (p ¼ 0.036), which was attributed to design differences of central stabilizers featuring both implants. A conversion of rotary motions into relative micromotions, using recently measured moments acting on these implants during daily activities of living, nullified this difference (p ¼ 0.088). However, depending on the shoulder load case considered, a clear difference appeared (p ¼ 0.031-0.045). In conclusion, both resurfacing implants are capable of achieving sufficient initial fixation on the humeral head and perform relative micromotions in a range considered safe for bony ingrowth. Patient-related parameters do not appear to influence the initial fixation of these implants. Keywords: arthroplasty; shoulder; cementless resurfacing; initial fixation; osseointegrationIn the 1950s, Neer 1 established modern shoulder arthroplasty and revolutionized its design with his perception of ''as near as possible to the anatomy.'' Since then designs have undergone many developments, as use of shoulder replacements has increased over the past 40 years. 2 The latest development is towards cementless surface replacement arthroplasties (CSRAs). Their positioning is independent of the humeral cavity and therefore easy to handle in the treatment of patients who have extra-articular deformities that make the placement of a conventional stemmed prosthesis difficult or impossible. 3,4 The design of CSRAs provides a primary press-fit, and in the majority of cases HA coatings should allow for accelerated biological fixation. 5 The benefit is minimal bone loss that facilitates later revision with a conventional total shoulder arthroplasty. 6,7 However, as promising as the implantation of a CSRA seems to be, a recent clinical study showed a high revision rate, 22% after 13 mos due to pain that may be caused by overstuffing of the joint, not resurfacing the glenoid, or loosening of the humeral component. 8 The complication rates in previous studies seem comparable and vary between 3% and 20%. 5,9-11 A further challenge is recognizing aseptic loosening in standard radiographs, since the bony bearing is always occulted by the metal concavity.As we have learned from total hip arthroplasty, initial fixation is the prerequisite for a good long-term success of cementless implants. This largely depends on how tightly the implant is fixed to the bone. 12 Relative micromotions ...

Resurfacing of the humeral head: An analysis of the bone stock and osseous integration under the implant

“…None of the implants showed connective tissue formation at larger proportions of the interface between the implant surface and bone. This suggests sufficient primary stability and is in line with in‐vitro biomechanical CSRA studies …”

“…However, in this study no significant differences between the crown and stem fixed geometries or the coatings of the four manufacturers were observed. Similarly, Jacubowitz et al reported in their biomechanical study only about small differences in the micromotions when evaluating the primary stability of the crown and stemmed fixed geometries …”

“…To date, several studies have reported satisfying short‐ and mid‐term results , . Furthermore, an in vitro study evaluated the primary biomechanical stability of CSRA and found that the implants were providing sufficient initial fixation on the humeral head . This is confirmed by the short term results of the Danish shoulder arthroplasty registry where the cumulative failure mode after 5 years was 9.9% and was mainly caused by glenoid attrition (28.6%) and rotator cuff dysfunction (20.6%) whereas loosening only accounted for 3.2% .…”

“…Due to the radiopaque implant shell that covers the bearing bone stock, the implant interface is not visible on standard x‐rays . At an early stage this makes it difficult to recognize failure of osseous integration or aseptic loosening using standard radiographs or computed tomography . Considering that hip resurfacing arthroplasty (HRA), which has the same design philosophy, shows distinct patterns of bone remodelling under the femoral cup, it is of interest to investigate the CSRA implants.…”

Thermal damage during humeral reaming in total shoulder resurfacing

“…Adequate fixation to the host bone of the proximal humerus is a critical factor in the success of resurfacing. [13] Glenoid loosening is a significant factor in failure of total shoulder replacement,[14] with the majority of revision arthroplasty performed to address this issue. [15] High temperatures have been noted at the implant glenoid interface[16] and glenoid reaming has been shown to generate high temperatures in vivo , beyond the physiological threshold required to induce osteonecrosis.…”