Analysis of severely fractured glenoid components: clinical consequences of biomechanics, design, and materials selection on implant performance

Reverse total shoulder arthroplasty baseplate stability with locking vs. non-locking peripheral screws

BACKGROUND

There are many options for glenosphere baseplate fixation commercially available, yet there is little biomechanical evidence supporting one type of fixation over another. In this study, we compared the biomechanical fixation of a reverse total shoulder glenoid baseplate secured with locking or non-locking peripheral screws.

METHODS

Both a non-augmented mini baseplate with full backing support and an augmented baseplate were testing after implantation in solid rigid polyurethane foam. Each baseplate was implanted with a 30 mm central compression screw and four peripheral screws, either locking or non-locking (15 mm anterior/posterior and 30 mm superior/inferior). A 1 Hz cyclic force of 0-750 N was applied at a 60^o angle for 5000 cycles. Throughout the test, the displacement of the baseplate was measured using a 3D Digital Image Correlation System.

FINDINGS

The amount of migration measured in the both the non-augmented and augment cases shows no significant differences between locking and non-locking cases at the final cycle count (non-augment: 5.66 +/- 2.29 μm vs. 3.71 +/- 1.23 μm; p = 0.095, augment: 15.43 +/- 8.49 μm vs. 12.46 +/- 3.24 μm; p = 0.314). Additionally, the amount of micromotion measured for both sample types shows the same lack of significant difference (non-augment: 10.79 +/- 5.22 μm vs. 10.16 +/- 7.61 μm; p = 0.388, augment: 55.03 +/- 10.13 μm vs. 54.84 +/- 10.65 μm; p = 0.968).

INTERPRETATION

The presence of locking versus non-locking peripheral screws does not make a significant difference on the overall stability of a glenoid baseplate, in both a no defect case with a non-augmented baseplate and a bone defect case with an augmented baseplate.

Synergy between vitamin E and D-sorbitol in enhancing oxidation stability of highly crosslinked ultrahigh molecular weight polyethylene

Oxidative stability of radiation crosslinked ultrahigh molecular weight polyethylene (UHMWPE) artificial joints is significantly improved by vitamin E (VE), but there is a dilemma that VE hinders crosslinking and thus jeopardizes the wear of UHMWPE. In this effort, we proposed an efficient strategy to stabilize UHMWPE under limited antioxidant contents, where VE and D-sorbitol (DS) were used as the primary antioxidant and the secondary antioxidant respectively. For non-irradiated blends with fixed antioxidant contents, oxidative stability accessed by oxidation induction time (OIT) of VE/DS/UHMWPE blends was superior to that of VE/UHMWPE blends, while DS/UHMWPE blends showed no increase in OIT. The cooperation between DS and VE exhibited a synergistic effect on enhancing the oxidative stability of UHMWPE. Interestingly, the irradiated VE/DS/UHMWPE blends showed comparable OIT but a significantly higher crosslink density than the irradiated VE/UHMWPE blends. The crystallinity, melting point, and in vitro biocompatibility of the blends were not affected by VE and DS. The quantitative relationships of mechanical properties, oxidation stability, crystallinity and crosslink density were established to unveil the correlation of these key factors. The overall properties of VE/UHMWPE and VE/DS/UHMWPE blends were compared to elucidate the superiority of the antioxidant compounding strategy. These findings provide a paradigm to break the trade-off between oxidative stability, crosslink density and mechanical properties, which is constructive to develop UHMWPE bearings with upgraded performance for total joint replacements. STATEMENT OF SIGNIFICANCE: VE-stabilized UHMWPE is the most commonly used material in total joint replacements at present. However, oxidation and wear resistance of VE/UHMWPE implants cannot be unified since VE reduces the efficiency of radiation crosslinking. It limits the use of VE. Herein, we proposed a compounding stabilization by the synergy between VE and DS. The antioxidation capability of VE was revived by DS, thus enhancing the oxidation stability of unirradiated UHMWPE. The irradiated VE/DS/UHMWPE exhibited similar oxidation stability but higher crosslink density than irradiated VE/UHMWPE, which is beneficial to combat wear of UHMWPE and to inhibit the occurrence of osteolysis. This synergistic antioxidation strategy endows the UHMWPE joint material with good overall performance, which is of clinical significance.

Effect of material selection on tibial post stresses in posterior-stabilized knee prosthesis

Aims

The material and design of knee components can have a considerable effect on the contact characteristics of the tibial post. This study aimed to analyze the stress distribution on the tibial post when using different grades of polyethylene for the tibial inserts. In addition, the contact properties of fixed-bearing and mobile-bearing inserts were evaluated.

Methods

Three different grades of polyethylene were compared in this study; conventional ultra high molecular weight polyethylene (UHMWPE), highly cross-linked polyethylene (HXLPE), and vitamin E-stabilized polyethylene (VEPE). In addition, tibial baseplates with a fixed-bearing and a mobile-bearing insert were evaluated to understand differences in the contact properties. The inserts were implanted in neutral alignment and with a 10° internal malrotation. The contact stress, von Mises stress, and equivalent plastic strain (PEEQ) on the tibial posts were extracted for comparison.

Results

The stress and strain on the tibial post for the three polyethylenes greatly increased when the insert was placed in malrotation, showing a 38% to 56% increase in von Mises stress and a 335% to 434% increase in PEEQ. The VEPE insert had the lowest PEEQ among the three materials. The mobile-bearing design exhibited a lower increase in stress and strain around the tibial posts than the fixed-bearing design.

Conclusion

Using VEPE for the tibial component potentially eliminates the risk of material permanent deformation. The mobile-bearing insert can help to avoid a dramatic increase in plastic strain around the tibial post in cases of malrotation. The mobility allows the pressure to be distributed on the tibial post and demonstrated lower stresses with all three polyethylenes simulated.

Cite this article: Bone Joint Res 2020;9(11):768–777.

The effect of vitamin E-enhanced cross-linked polyethylene on wear in shoulder arthroplasty-a wear simulator study

BACKGROUND

Wear of the polyethylene glenoid component and subsequent particle-induced osteolysis remains one of the most important modes of failure of total shoulder arthroplasty. Vitamin E is added to polyethylene to act as an antioxidant to stabilize free radicals that exist as a byproduct of irradiation used to induce cross-linking. This study was performed to assess the in vitro performance of vitamin E-enhanced polyethylene compared with conventional polyethylene in a shoulder simulator model.

METHODS

Vitamin E-enhanced, highly cross-linked glenoid components were compared with conventional ultrahigh-molecular-weight polyethylene glenoids, both articulating with a ceramic humeral head component using a shoulder joint simulator over 500,000 cycles. Unaged and artificially aged comparisons were performed. Volumetric wear was assessed by gravimetric measurement, and wear particle analysis was also subsequently performed.

RESULTS

Vitamin E-enhanced polyethylene glenoid components were found to have significantly reduced wear rates compared with conventional polyethylene in both unaged (36% reduction) and artificially aged (49% reduction) comparisons. There were no differences detected in wear particle analysis between the 2 groups.

CONCLUSION

Vitamin E-enhanced polyethylene demonstrates improved wear compared with conventional polyethylene in both unaged and artificially aged comparisons and may have clinically relevant benefits.

A Study of the Biomechanical Behavior of the Implantation Method of Inverted Shoulder Prosthesis (BIO⁻RSA) under Different Abduction Movements

The shoulder is the most mobile joint of the human body, but it is very fragile; several pathologies, and especially muscular degenerations in the elderly, can affect its stability. These are more commonly called rotator cuff fractures. In the case of this type of pathology, the mobility of the shoulder decreases and pain appears. In order to restore mobility and reduce pain, implantation of an inverted shoulder prosthesis is recommended. Unfortunately, over time a notch phenomenon has been observed. In the lower position of the arm, part of the implant comes into contact with the scapula and therefore causes deterioration of the bone. Among the solutions adopted is the lateralized method with bone grafting. However, a main disadvantage of this method concerns the reconstruction of the graft in the case of prosthesis revision. In this context, the aim of the present work was to reconstruct the shoulder joint in 3D in order to obtain a bio-faithful geometry, and then study the behavior of different types of biomaterials that can replace bone grafting. To this end, three arm abduction motions were examined for three individuals. From the results obtained, it appears that grafts in ultra-high molecular weight polyethylene (UHMWPE) exhibit a behavior closer to that of bones.

Elastoplastic behavior of highly cross-linked and vitamin E-stabilized polyethylene - A biomechanical study

BACKGROUND

Vitamin E-stabilized cross-linked polyethylene has been touted to alleviate the negative effects of oxidation. Although it has demonstrated significant improvements in wear resistance, bio-tribology, and oxidative resistance, little is known about the effect of antioxidants and dosage of cross-linking on the mechanical strength. This study aimed to evaluate the mechanical properties of these novel materials, which are commonly used in orthopedic implants.

METHODS

Samples of different polymers were prepared with various levels of cross-linking and with or without vitamin E-stabilization and then tested according to ASTM D695 and D638. The elastoplastic characteristics under compression and tension were compared between the groups.

FINDINGS

Vitamin E-stabilized cross-linked polyethylene showed a significant increase in elastic modulus over other groups, with a maximum increase of 26% in compression and 40% in tension when compared to the highly cross-linked group without vitamin E stabilization. The elastoplastic behavior under compression differed to that in tension for all polymers, demonstrating the anisotropic characteristics of these polymers.

INTERPRETATION

The lower mechanical strength of highly cross-linked polyethylene has been a complication with the use of this polymer in orthopedic liners. This current study suggests that vitamin E-stabilized cross-linked polyethylene could be a suitable alternative material for knee implants because of its improved strength in resisting external forces.

What can be learned from an analysis of 215 glenoid component failures?

BACKGROUND

Glenoid component failure is a prevalent mechanical complication of anatomic total shoulder arthroplasty. The objective of this study was to identify surgeon-controlled factors that may be addressed to reduce the rate of glenoid component failure that is sufficiently symptomatic to merit surgical revision.

METHODS

We reviewed the clinical and radiographic features of 215 total shoulder arthroplasties that we revised for symptomatic glenoid component failure.

RESULTS

Glenoid component failure was associated with poor patient self-assessed shoulder function (mean Simple Shoulder Test score, 3.0 ± 2.7). These shoulders often showed multiple failure modes; 72% had glenoid component loosening, 69% had polyethylene wear, 51% had glenohumeral decentering, and 25% had humeral component loosening. Metal-backed/hybrid and keeled glenoid designs had higher rates of loosening (P = .010), malposition (P = .007), dislocation (P < .001), and early failure (P = .044) in comparison to pegged designs. Glenoid components with cement on the backside were more prevalent among those revised sooner than 5 years after the index surgery (P < .001).

CONCLUSIONS

Glenoid component failure remains a major cause of poor patient outcomes after total shoulder arthroplasty. The occurrence of severe glenoid component failure might be reduced by paying attention to glenoid component design and insertion technique, restoring the normal balance of the humeral head in the center of the glenoid, and considering a reverse total shoulder when the shoulder is unstable because of soft tissue deficiency.