How much bone support does an anatomic glenoid component need?

Abstract

Background: An important reason for failure of anatomic total shoulder arthroplasty is glenoid component loosening. We investigated the effect of backside bone support on the risk of failure of a glenoid component. Methods: A finite element model was developed. Virtual surgery was performed for 2 types of glenoid components (cemented all polyethylene [PE] vs. metal backed [MB]), both with gradually decreasing backside bone support. Both bone failure and fixation failure were analyzed. The percentages of bone failure and fixation failure in terms of the critical cement volume (CCV) and micromotion-threshold percentage ratio (MTPR) for the PE and MB components, respectively, were defined and compared. Results: For the reference PE model, the percentages of bone failure and fixation failure (CCV) were 17% and 34%, respectively. With eccentric loading for the MB component, the percentages of bone failure and fixation failure (MTPR) were 6% and 3%, respectively. A global increase in failure was observed with decreasing bone support. The increase in fixation failure, starting from the reference values (MTPR vs. CCV), was relatively more pronounced for the MB component (136% vs. 128%). Discussion: Decreasing backside bone support for an anatomic glenoid component leads to an increased risk of fixation and bone failure. For PE components, decreasing backside support to 95% bone support had only a limited effect. In the case of an MB component, we noticed an increase in micromotion and bone failure already starting from 97% bone support. We conclude that an anatomic glenoid component should always be implanted while maximizing backside bone support.

Mechanical behavior of hybrid glenoid components compared to all-PE components: a finite element analysis

“…Cemented all-PE components have lower revision rates in comparison to uncemented components, but when comparing rates of loosening or lysis at a follow-up of 5 years, there is no difference between cemented and uncemented components (1.1%) [ 8 ]. Although the cemented all-PE component is associated with adequate initial fixation and stability [ 23 ], fatigue can lead to micro-cracks and ultimate failure of the cemented fixation [ 7 ]. Therefore, the lower stresses in cement when using hybrid components are advantageous since it may lead to reduced risks of cement overloading.…”

“…Since these implants offer the potential of long-term stability through biologic fixation, they are considered promising alternatives to cemented all-PE components. Their success depends on sufficient initial stability to promote osseointegration, which can be problematic in patients with poor bone quality [ 7 ]. The mean %CCV in the present study ranged from 32.5% to 44.5% for all components, indicating possible failure due to fatigue.…”

“…3 ). All components were modeled as linear isotropic materials [ 7 ]: Polyethylene (ultra-high molecular weight polyethylene (UHMWPE); E, 361 mPa; Poisson ratio, 0.4101 m, ISO 5834–1&2); Titanium (Ti6Al4V-ELI; E, 112.4 gPa; Poisson ratio, 0.34) [ 10 ]; Cement (E, 2.0 gPa; Poisson ratio, 0.3) [ 3 ].

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“…Second, failure initiation in the cement mantle started if cement stresses exceeded 4 mPa. The critical cement volume (CCV) [ 7 ] was calculated to quantify the percentage of cement volume that exceeded the 4 mPa threshold.…”

“…Despite numerous attempts to improve fixation of TSA glenoid components [ 4 ], glenoid loosening remains a frequent cause of failure [ 1 , 5 , 6 ]. Most failures of all-PE components are attributed to debris caused by micro-cracks in the cement mantle due to fatigue, whereas failures of porous metal-backed PE components are attributed to micromotion exceeding thresholds that inhibit bony in-growth [ 7 ]. According to a recent review of joint registry data [ 8 ], metal-backed PE components have higher risks of revision, compared to cemented all-PE components.…”