Bone-cement interface of the glenoid component: stress analysis for varying cement thickness

Implant breakage after shoulder arthroplasty: a systematic review of data from worldwide arthroplasty registries and clinical trials

BACKGROUND

Implant breakage after shoulder arthroplasty is a rare complication after aseptic loosening, infection or persistent pain, resulting in malfunction of the components requiring revision surgery. This correlates with a high burden for the patient and increasing costs. Specific data of complication rates and implant breakage are available in detailed arthroplasty registries, but due to the rare occurrence and possibly underestimated value rarely described in published studies. The aim of this systematic review was to point out the frequency of implant breakage after shoulder arthroplasty. We hypothesized that worldwide arthroplasty registry datasets record higher rates of implant breakage than clinical trials.

METHODS

PubMed, MEDLINE, EMBASE, CINHAL, and the Cochrane Central Register of Controlled Trials database were utilized for this systematic review using the items "(implant fracture/complication/breakage) OR (glenoid/baseplate complication/breakage) AND (shoulder arthroplasty)" according to the PRISMA guidelines on July 3rd, 2023. Study selection, quality assessment, and data extraction were conducted according to the Cochrane standards. Case reports and experimental studies were excluded to reduce bias. The breakage rate per 100,000 observed component years was used to compare data from national arthroplasty registries and clinical trials, published in peer-reviewed journals. Relevant types of shoulder prosthetics were analyzed and differences in implant breakage were considered.

RESULTS

Data of 5 registries and 15 studies were included. Rates of implant breakage after shoulder arthroplasty were reported with 0.06-0.86% in registries versus 0.01-6.65% in clinical studies. The breakage rate per 100,000 observed component years was 10 in clinical studies and 9 in registries. There was a revision rate of 0.09% for registry data and 0.1% for clinical studies within a 10-year period. The most frequently affected component in connection with implant fracture was the glenoid insert.

CONCLUSION

Clinical studies revealed a similar incidence of implant failure compared to data of worldwide arthroplasty registries. These complications arise mainly due to breakage of screws and glenospheres and there seems to be a direct correlation to loosening. Periprosthetic joint infection might be associated with loosening of the prosthesis and subsequent material breakage. We believe that this analysis can help physicians to advise patients on potential risks after shoulder arthroplasty.

LEVEL OF EVIDENCE

III.

An experimental study on the impact of prosthesis temperature on the biomechanical properties of bone cement fixation

PURPOSE

To investigate the effect of the femoral component and tibial plateau component temperature on the strength of cement fixation during total knee arthroplasty (TKA).

METHODS

Femoral prosthesis, tibial plateau prosthesis, and polypropylene mold base were used to simulate TKA for bone cement fixation. Pre-cooling or pre-warming of femoral and tibial plateau components at different temperatures (4 °C, 15 °C, 25 °C, 37 °C, 45 °C), followed by mixing and stirring of bone cement at laboratory room temperature (22 °C), were performed during research. The prosthesis and the base adhered together, and the bone cement was solidified for 24 h at a constant temperature of 37 °C to verify the hardness of the bone cement with a push-out test.

RESULTS

The push-out force of the femoral prosthesis after fixation was higher than that of the tibial plateau prosthesis, and with the increase of the prosthesis temperature, the push-out force after fixation of the bone cement also increased linearly and the porosity of the prosthetic cement in the tibia and femur decreased as the temperature increased.

CONCLUSION

Without changing the mixing temperature and solidification temperature, the fixation strength of the femoral prosthesis is higher than that of the tibial plateau prosthesis. Properly increasing the temperature of the prosthesis can increase the push-out force of the fixation strength.

Impact of polyethylene glenoid cementation technique on cement mantle integrity and stability after cyclic loading: a computed tomography and biomechanical study

BACKGROUND

There are no generally accepted guidelines for polyethylene (PE) glenoid component cementation techniques. In particular, it is not known whether the backside of a PE glenoid should be fully or partially cemented-or not cemented at all. We hypothesized that cementing techniques would have an impact on cement mantle volume and integrity, as well as biomechanical stability, measured as micromotion under cyclic loading.

METHODS

To address our hypothesis, 3 different cementation techniques using a single 2-peg PE glenoid design with polyurethane foam were compared regarding (1) the quality and quantity of the cement mantle and (2) biomechanical stability after cyclic loading in vitro. Eight identically cemented glenoids per group were used. Group A underwent cement application only into the peg holes, group B received additional complete cement mantle application on the backside of the glenoid, and group C received the same treatment as group B but with additional standardized drill holes in the surface of the glenoid bone for extra cement interdigitation. All glenoids underwent cyclic edge loading by 10⁵ cycles according to ASTM F2028-14. Before and after loading, cement mantle evaluation was performed by XtremeCT and biomechanical strength and loosening were evaluated by measuring the relative motion of the implants.

RESULTS

The cement mantle at the back of the implant was incomplete in group A as compared with groups B and C, in which the complete PE backside was covered with a homogeneous cement mantle. The cement mantle was thickest in group C, followed by group B (P = .006) and group A (P < .001). We did not detect any breakage of the cement mantle in any of the 3 groups after testing. Primary stability during cyclic loading was similar in all groups after the "running-in" phase (up to 4000 cycles). Gross loosening did not occur in any implant.

CONCLUSIONS

Coverage of the PE glenoid with cement was reproducible in the fully cemented groups (ie, groups B and C) as compared with relevant cement defects in group A. The addition of cement to the back of the PE glenoid and additional drill holes in the glenoid surface did not improve primary stability in the tested setting.

Quantitative statistical shape model-based analysis of humeral head migration, Part 2: Shoulder osteoarthritis

We wanted to investigate the quantitative characteristics of humeral head migration (HHM) in shoulder osteoarthritis (OA) and their possible associations with scapular morphology. We quantified CT-scan-based-HHM in 122 patients with a combination of automated 3D scapulohumeral migration (=HHM with respect to the scapula) and glenohumeral migration (=HHM with respect to the glenoid) measurements. We divided OA patients in Group 1 (without HHM), Group 2a (anterior HHM) and Group 2b (posterior HHM). We reconstructed and measured the prearthropathy scapular anatomy with a statistical shape model technique. HHM primarily occurs in the axial plane in shoulder OA. We found "not-perfect" correlation between subluxation distance AP and scapulohumeral migration values (r_s  = 0.8, p < 0.001). Group 2b patients had a more expressed prearthropathy glenoid retroversion (13° vs. 7°, p < 0.001) and posterior glenoid translation (4 mm vs. 6 mm, p = 0.003) in comparison to Group 1. Binary logistic regression analysis indicated prearthropathy glenoid version as a significant predictor of HHM (χ² = 27, p < 0.001). Multivariate regression analysis showed that the pathologic version could explain 56% of subluxation distance-AP variance and 75% of the scapulohumeral migration variance (all p < 0.001). Herewith, every degree increase in pathologic glenoid retroversion was associated with an increase of 1% subluxation distance-AP, and scapulohumeral migration. The occurrence of posterior HHM is associated with prearthropathy glenoid retroversion and more posterior glenoid translation. The reported regression values of HHM in the function of the pathologic glenoid version could form a basis toward a more patient-specific correction of HHM.

Effects of conform, non-conform, and hybrid conformity toward stress distribution at the glenoid implant and cement: A finite element study

Glenoid conformity is one of the important aspects that could contribute to implant stability. However, the optimal conformity is still being debated among the researchers. Therefore, this study aims to analyze the stress distribution of the implant and cement in three types of conformity (conform, non-conform, and hybrid) in three load conditions (central, anterior, and posterior). Glenoid implant and cement were reconstructed using Solidwork software and a 3D model of scapula bone was done using MIMICS software. Constant load, 750 N, was applied at the central, anterior, and posterior region of the glenoid implant which represents average load for daily living activities for elder people, including, walking with a stick and standing up from a chair. The results showed that, during center load, an implant with dual conformity (hybrid) showed the best (Max Stress-3.93 MPa) and well-distributed stress as compared to other conformity (Non-conform-7.21 MPa, Conform-9.38 MPa). While, during eccentric load (anterior and posterior), high stress was located at the anterior and posterior region with respect to the load applied. Cement stress for non-conform and hybrid implant recorded less than 5 MPa, which indicates it had a very low risk to have cement microcracks, whilst, conform implant was exposed to microcrack of the cement. In conclusion, hybrid conformity showed a promising result that could compromise between conform and non-conform implant. However, further enhancement is required for hybrid implants when dealing with eccentric load (anterior and posterior).

The Application of Digital Volume Correlation (DVC) to Evaluate Strain Predictions Generated by Finite Element Models of the Osteoarthritic Humeral Head

Continuum-level finite element models (FEMs) of the humerus offer the ability to evaluate joint replacement designs preclinically; however, experimental validation of these models is critical to ensure accuracy. The objective of the current study was to quantify experimental full-field strain magnitudes within osteoarthritic (OA) humeral heads by combining mechanical loading with volumetric microCT imaging and digital volume correlation (DVC). The experimental data was used to evaluate the accuracy of corresponding FEMs. Six OA humeral head osteotomies were harvested from patients being treated with total shoulder arthroplasty and mechanical testing was performed within a microCT scanner. MicroCT images (33.5 µm isotropic voxels) were obtained in a pre- and post-loaded state and BoneDVC was used to quantify full-field experimental strains (≈ 1 mm nodal spacing, accuracy = 351 µstrain, precision = 518 µstrain). Continuum-level FEMs with two types of boundary conditions (BCs) were simulated: DVC-driven and force-driven. Accuracy of the FEMs was found to be sensitive to the BC simulated with better agreement found with the use of DVC-driven BCs (slope = 0.83, r² = 0.80) compared to force-driven BCs (slope = 0.22, r² = 0.12). This study quantified mechanical strain distributions within OA trabecular bone and demonstrated the importance of BCs to ensure the accuracy of predictions generated by corresponding FEMs.

How much bone support does an anatomic glenoid component need?

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.

The effect of glenohumeral radial mismatch on different augmented total shoulder arthroplasty glenoid designs: a finite element analysis

BACKGROUND

Augmented glenoid implants to correct bone loss can possibly reconcile current prosthetic failures and improve long-term performance for total shoulder arthroplasty. Biomechanical implant studies have suggested benefits from augmented glenoid components, but limited evidence exists on optimal design.

METHODS

An integrated kinematic finite element analysis (FEA) model was used to evaluate optimal augmented glenoid design based on biomechanical performance in translation in the anteroposterior plane similar to clinical loading and failure mechanisms with osteoarthritis. Computer-aided design software models of 2 different commercially available augmented glenoid designs-wedge (Equinox; Exactech, Inc., Gainesville, FL, USA) and step (STEPTECH; DePuy Synthes, Warsaw, IN, USA) were created according to precise manufacturer's dimensions of the implants. Using FEA, they were virtually implanted to correct 20° of retroversion. Two glenohumeral radial mismatches, 3.5/4 mm and 10 mm, were evaluated for joint stability and implant fixation simulating high-risk conditions for failure.

RESULTS

The wedged and step designs showed similar glenohumeral joint stability under both radial mismatches. Surrogate for micromotion was a combination of distraction, translation, and compression. With similar behavior and measurements for distraction and translation, compression dictated micromotion (wedge: 3.5 mm = 0.18 mm and 10 mm = 0.10 mm; step: 3.5 mm = 0.19 mm and 10 mm = 0.25 mm). Stress levels on the backside of the implant and on the cement mantle were higher using a step design.

DISCUSSION

Greater radial mismatch has the advantage of providing higher glenohumeral stability with tradeoffs, such as higher implant and cement mantle stress levels, and micromotion worse when using a step design.

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.

Does the cement mantle thickness influence the glenoid loosening in anatomic total shoulder arthroplasty? An experimental study

BACKGROUND

Glenoid component loosening is the most frequent failure mode. Few data are available on the effect of thickness of cement on glenoid loosening. The purpose of this study is to determine if the cement mantle thickness influences the mode and localization of loosening. Our hypotheses are: 1) failure is caused by traction stresses generated within the cement mantle and 2) a thicker cement mantle amplifies the rocking horse effect.

METHODS

Using bone substitute, an experimental protocol was designed to compare loosening of a keeled glenoid prosthesis in axial traction and off-centered-load, to recreate the rocking-horse effect (1.000.000 cycles). Different standardized mantle of cement between the back of the glenoid and the foam were tested (0-1 - 2-3 mm). The displacement of the polyethylene was assessed with an LVDT (Linear Variable Differential Transformer) gauge when the prosthetic humeral head loaded the opposite part of the implant.

RESULTS

The loosening took place within the keel of the implant, and at the polyethylene-cement interface in traction if there was cement at the back of the polyethylene. For cycling loading, we observed a loosening at this interface, with associated fracture of the cement, only for cement 2 and 3 mm thick.

CONCLUSION

This experimental study suggests that the cement mantle should be as thin as possible between the back of the implant and the sub-chondral bone but should be optimized around the keel of the implant.

LEVEL OF EVIDENCE

Basic Science Study.

Polyethylene glenoid component fixation geometry influences stability in total shoulder arthroplasty

Glenoid component stability is essential to ensure successful long-term survivability following total shoulder arthroplasty. As such, this computational study assessed the stability of five all-polyethylene glenoid components (Keel, Central-Finned 4-Peg, Peripheral 4-Peg, Cross-Keel, and Inverted-Y), using simulated joint loading in an osteoarthritic patient cohort. Stability was assessed on the basis of component micromotion in the tangential and normal directions. Maximum tangential micromotion occurred in the Cross-Keel (146 ± 46 µm), which was significantly greater (p < .001) than the other components. Maximum normal micromotion occurred in the Inverted-Y (109 ± 43 µm), which was significantly greater (p ≤ .002) than the other four components. In general, the Central-Finned 4-Peg exhibited the least normal and tangential micromotion, while the keeled components shown the highest normal and tangential micromotion. This study suggests that modifications to keeled designs do not improve component stability under the conditions tested, and pegged components show superior computational stability.

Biomechanical comparison of glenoid implants with adaptable and fixed backside curvatures in anatomic total shoulder arthroplasty

BACKGROUND

We evaluated the biomechanical effects and potential advantages of glenoid implants with adaptable backside curvature radii and compared them with standard implants having fixed backside curvatures in anatomic total shoulder arthroplasty (aTSA) for primary glenohumeral osteoarthritis with uniconcave glenoids.

METHODS

A glenoid implant with adaptable backside curvatures (Aequalis PerFORM, Tornier SAS, Montbonnot, France) was compared with its previous model having a fixed curvature radius. Virtual aTSAs were performed in 24 patients from preoperative shoulder computed tomography data sets, using both implants in each patient. For all 48 simulated aTSAs, we first measured the glenoid bone reaming depth, subchondral bone quality after reaming, and implant backside surface and then the predicted cement stress, bone-cement interfacial stress, and bone strain at 60° of arm abduction. These biomechanical quantities were tested for differences between adaptable and fixed implants and for correlations between preoperative measurements and postoperative predictions.

RESULTS

Adaptable glenoid implants induced a significant decrease in cement stress (P = .008), bone-cement interfacial stress (P = .045), and bone strain (P = .039), particularly for glenoids with curvature radii larger than 40 mm. However, these biomechanical effects were not significantly correlated with an increase in subchondral glenoid bone quality.

CONCLUSIONS

Our study confirms the presumed biomechanical advantages of adaptable glenoid implants, even though the effects were not directly due to the adaptation of the backside curvature radius. Benefits were more pronounced for glenoids with large curvature radii. Our initial biomechanical findings should now be corroborated with large-scale clinical studies.

Evaluation of thirty eight cemented pegged glenoid components with variable backside curvature: two-year minimum follow-up

BACKGROUND

The PERFORM™ pegged glenoid system has been used for shoulder arthroplasty since 2012. This system offers multiple backside curvatures per size to better match variable patient anatomy. As a result, less reaming is required and subchondral bone is preserved-a critical factor in preventing glenoid migration and loosening, thus enhancing implant longevity.

PURPOSE

The purpose of this study was to analyze all radiographic modifications around this new glenoid implant.

METHOD

Thirty-eight shoulders which received the PERFORM™ pegged glenoid component between June 2012 and January 2014 for primary or secondary osteoarthritis were reviewed at two-years minimum follow-up. There were 13 men and 22 women with an average age of 67 years. Humeral components were an uncemented short stem implant in nine (23%) and a resurfacing implant in 29 (77%).

RESULTS

At 27-months average follow-up (24-41), Constant score improved from 30 to 65 points. Range of motion improved significantly at follow-up from 100° to 142° for the anterior elevation, and from 15 to 40° for the external rotation. Radiographic lucent lines (RLL) were observed post-operatively in eight cases (21%), and in 16 cases (42%) at the last follow-up with an increase of the RLL score from 0.36 ± 0.8 to 1.3 ± 2 (p < 0.001) without signs of loosening (RLL > 12). One revision has been performed after anterior shoulder dislocation, rotator cuff tear and glenoid component migration. RLL score was not correlated with dominant side, sex, age, or Constant score.

DISCUSSION-CONCLUSION

The cemented pegged glenoid component with multiple backside curvatures gave satisfactory results at two-years minimum follow-up for up to three years with a low RLL score. Long-term studies are mandatory to confirm these results.

Cement stress predictions after anatomic total shoulder arthroplasty are correlated with preoperative glenoid bone quality

HYPOTHESIS

We hypothesized that biomechanical parameters typically associated with glenoid implant failure after anatomic total shoulder arthroplasty (aTSA) would be correlated with preoperative glenoid bone quality.

METHODS

We developed an objective automated method to quantify preoperative glenoid bone quality in different volumes of interest (VOIs): cortical bone, subchondral cortical plate, subchondral bone after reaming, subchondral trabecular bone, and successive layers of trabecular bone. Average computed tomography (CT) numbers (in Hounsfield units [HU]) were measured in each VOI from preoperative CT scans. In parallel, we built patient-specific finite element models of simulated aTSAs to predict cement stress, bone-cement interfacial stress, and bone strain around the glenoid implant. CT measurements and finite element predictions were obtained for 20 patients undergoing aTSA for primary glenohumeral osteoarthritis. We tested all linear correlations between preoperative patient characteristics (age, sex, height, weight, glenoid bone quality) and biomechanical predictions (cement stress, bone-cement interfacial stress, bone strain).

RESULTS

Average CT numbers gradually decreased from cortical (717 HU) to subchondral and trabecular (362 HU) bone. Peak cement stress (4-10 MPa) was located within the keel hole, above the keel, or behind the glenoid implant backside. Cement stress, bone-cement interfacial stress, and bone strain were strongly negatively correlated with preoperative glenoid bone quality, particularly in VOIs behind the implant backside (subchondral trabecular bone) but also in deeper trabecular VOIs.

CONCLUSION

Our numerical study suggests that preoperative glenoid bone quality is an important parameter to consider in aTSA, which may be associated with aseptic loosening of the glenoid implant. These initial results should now be confronted with clinical and radiologic outcomes.

Finite element models of the human shoulder complex: a review of their clinical implications and modelling techniques

The human shoulder is a complicated musculoskeletal structure and is a perfect compromise between mobility and stability. The objective of this paper is to provide a thorough review of previous finite element (FE) studies in biomechanics of the human shoulder complex. Those FE studies to investigate shoulder biomechanics have been reviewed according to the physiological and clinical problems addressed: glenohumeral joint stability, rotator cuff tears, joint capsular and labral defects and shoulder arthroplasty. The major findings, limitations, potential clinical applications and modelling techniques of those FE studies are critically discussed. The main challenges faced in order to accurately represent the realistic physiological functions of the shoulder mechanism in FE simulations involve (1) subject-specific representation of the anisotropic nonhomogeneous material properties of the shoulder tissues in both healthy and pathological conditions; (2) definition of boundary and loading conditions based on individualised physiological data; (3) more comprehensive modelling describing the whole shoulder complex including appropriate three-dimensional (3D) representation of all major shoulder hard tissues and soft tissues and their delicate interactions; (4) rigorous in vivo experimental validation of FE simulation results. Fully validated shoulder FE models would greatly enhance our understanding of the aetiology of shoulder disorders, and hence facilitate the development of more efficient clinical diagnoses, non-surgical and surgical treatments, as well as shoulder orthotics and prosthetics. © 2016 The Authors. International Journal for Numerical Methods in Biomedical Engineering published by John Wiley & Sons Ltd.

Bone density and anisotropy affect periprosthetic cement and bone stresses after anatomical glenoid replacement: A micro finite element analysis

Glenoid loosening is still a main complication for shoulder arthroplasty. We hypothesize that cement and bone stresses potentially leading to fixation failure are related not only to glenohumeral conformity, fixation design or eccentric loading, but also to bone volume fraction, cortical thickness and degree of anisotropy in the glenoid. In this study, periprosthetic bone and cement stresses were computed with micro finite element models of the replaced glenoid depicting realistic bone microstructure. These models were used to quantify potential effects of bone microstructural parameters under loading conditions simulating different levels of glenohumeral conformity and eccentric loading simulating glenohumeral instability. Results show that peak cement stresses were achieved near the cement-bone interface in all loading schemes. Higher stresses within trabecular bone tissue and cement mantle were obtained within specimens of lower bone volume fraction and in regions of low anisotropy, increasing with decreasing glenohumeral conformity and reaching their maxima below the keeled design when the load is shifted superiorly. Our analyses confirm the combined influences of eccentric load shifts with reduced bone volume fraction and anisotropy on increasing periprosthetic stresses. They finally suggest that improving fixation of glenoid replacements must reduce internal cement and bone tissue stresses, in particular in glenoids of low bone density and heterogeneity.

Glenoid cement mantle characterization using micro-computed tomography of three cement application techniques

BACKGROUND

Numerous studies have documented the concern for progressive radiolucent lines, signifying debonding and subsequent aseptic loosening of the glenoid component. In this study, we compared 3 cementation methods to secure a central peg in 15 cadaveric glenoids.

METHODS

Cement application techniques consisted of (1) compression of multiple applications of cement using manual pressure over gauze with an Adson clamp, (2) compression of multiple applications of cement using a pressurizer device, and (3) no compression of a single application of cement. Each glenoid was then imaged with high-resolution micro-computed tomography and further processed by creating 3-dimensional computerized models of implant, bone, and cement geometry. Cement morphology characteristics were then analyzed in each of the models.

RESULTS

There were no significant differences detected between the 2 types of compression techniques; however, there was a significant difference between compression methods and use of no compression at all. All morphologic characteristics of a larger cement mantle were significantly correlated with greater cortical contact.

CONCLUSIONS

We demonstrate that compression techniques create a larger cement mantle. Increased size of the cement mantle is associated with increased contact with cortical bone at the glenoid vault. This method for characterizing the cement mantle by micro-computed tomography scanning techniques and 3-dimensional analysis may also be useful in future finite element analysis studies.

Peri-implant stress correlates with bone and cement morphology: Micro-FE modeling of implanted cadaveric glenoids

Aseptic loosening of cemented joint replacements is a complex biological and mechanical process, and remains a clinical concern especially in patients with poor bone quality. Utilizing high resolution finite element analysis of a series of implanted cadaver glenoids, the objective of this study was to quantify relationships between construct morphology and resulting mechanical stresses in cement and trabeculae. Eight glenoid cadavers were implanted with a cemented central peg implant. Specimens were imaged by micro-CT, and subject-specific finite element models were developed. Bone volume fraction, glenoid width, implant-cortex distance, cement volume, cement-cortex contact, and cement-bone interface area were measured. Axial loading was applied to the implant of each model and stress distributions were characterized. Correlation analysis was completed across all specimens for pairs of morphological and mechanical variables. The amount of trabecular bone with high stress was strongly negatively correlated with both cement volume and contact between the cement and cortex (r = -0.85 and -0.84, p < 0.05). Bone with high stress was also correlated with both glenoid width and implant-cortex distance. Contact between the cement and underlying cortex may dramatically reduce trabecular bone stresses surrounding the cement, and this contact depends on bone shape, cement amount, and implant positioning.

Effects of osteoarthritis on load transfer after cemented total shoulder arthroplasty

BACKGROUND

Total shoulder arthroplasty is commonly performed to treat glenohumeral osteoarthritis (OA); however, little is understood of the mechanics of the reconstructed OA shoulder. We sought to establish the effects of OA-induced changes in bone density and retroversion angle on load transfer and stress distribution in the bone-implant system of the scapula.

METHODS

We developed finite element models of reconstructed healthy and OA scapulas with a virtually implanted glenoid prosthesis design. For the OA scapula, models with uncorrected and corrected retroversion were created. Loads were applied at the center or posteriorly on the glenoid surface.

RESULTS

Our results suggest that with reconstruction of the corrected glenoid with a contemporary implant, cement stresses increase and the load transfer pattern changes with eccentric loads. The load transfer and local stresses in the bone-implant system in the retroverted glenoid are less sensitive to changes in loading location. Furthermore, the load transfer in the OA glenoid is less sensitive to the effect of peg proximity to the cortical shell than in the healthy glenoid.

CONCLUSION

We provided evidence of how load sharing is altered among healthy, corrected OA, and retroverted OA glenoids. We demonstrated that correction of retroversion in OA glenoids may actually increase the risk for stress shielding and cement failure compared with retroverted glenoids, and OA patients can accommodate shorter pegs because of the higher glenoid bone stiffness in the OA glenoid.

Load transfer after cemented total shoulder arthroplasty

BACKGROUND

Glenoid loosening is the primary reason for failure after a total shoulder arthroplasty (TSA), but the failure mechanism is not yet known. This study determined how the load transfer and stress distribution are affected by the introduction of a glenoid implant.

METHODS

We developed a finite-element model of a scapula with and without a virtually implanted modern glenoid prosthesis design. Two load magnitudes were considered: normal and high. Loading locations were simulated at the center and at 4 eccentric positions on the glenoid. A metal-backed implant was also simulated to understand the effect of fixation stiffness.

RESULTS

In the intact glenoid, for both center and eccentric loading, the majority of stress was distributed in the cancellous bone, whereas after a reconstruction, stresses in that region were lower. Metal-backed implants further decreased the joint load carried by the bone. Stresses in the cement layer increased during eccentric and high-magnitude loading.

CONCLUSION

This study provided a basic understanding of the load-sharing phenomenon after a TSA that could explain glenoid loosening failure. Our results suggest that with reconstruction of the glenoid with a contemporary implant, the load transfer pattern is significantly altered, with eccentric and high-magnitude loads increasing stresses in the cement indicating potential for failure. The use of a metal-backed implant reduces the load carried by the bone, which may be detrimental to long-term TSA survival.

Need for CT-based bone density modelling in finite element analysis of a shoulder arthroplasty revealed through a novel method for result analysis

Treatment of common pathologies of the shoulder complex, such as rheumatoid arthritis and osteoporosis, is usually performed by total shoulder arthroplasty (TSA). Survival of the glenoid component is still a problem in TSA, whereas the humeral component is rarely subject to failure. To set up a finite element analysis (FEA) for simulation of a TSA in order to gain insight into the mechanical behaviour of a glenoid implant, the modelling procedure and the application of boundary conditions are of major importance because the computed result strongly depends upon the accuracy and sense of realism of the model. The goal of this study was to show the influence on glenoid stress distribution of a patient-specific bone density distribution compared with a homogenous bone density distribution for the purpose of generating a valid model in future FEA studies of the shoulder complex. Detailed information on the integration of bone density properties using existing numerical models as well as the applied boundary conditions is provided. A novel approach involving statistical analysis of values derived from an FEA is demonstrated using a cumulative distribution function. The results show well the mechanically superior behaviour of a realistic bone density distribution and therefore emphasise the necessity for patient-specific simulations in biomechanical and medical simulations.

Glenoid implant orientation and cement failure in total shoulder arthroplasty: a finite element analysis

BACKGROUND

To minimize glenoid implant loosening in total shoulder arthroplasty (TSA), the ideal surgical procedure achieves correction to neutral version, complete implant-bone contact, and bone stock preservation. These goals, however, are not always achievable, and guidelines to prioritize their impact are not well established. The purpose of this study was to investigate how the degree of glenoid correction affects potential cement failure.

METHODS

Eight patient-specific computer models were created for 4 TSA scenarios with different permutations of retroversion correction and implant-bone contact. Two bone models were used: a homogeneous cortical bone model and a heterogeneous cortical-trabecular bone model. A 750-N load was simulated, and cement stress was calculated. The risk of cement mantle fracture was reported as the percentage of cement stress exceeding the material endurance limit.

RESULTS

Orienting the glenoid implant in retroversion resulted in the highest risk of cement fracture in a homogeneous bone model (P < .05). In the heterogeneous bone model, complete correction resulted in the highest risk of failure (P = .0028). A positive correlation (ρ = 0.901) was found between the risk of cement failure and amount of exposed trabecular bone.

CONCLUSIONS

Incorporating trabecular bone into the model changed the effect of implant orientation on cement failure. As exposed trabecular bone increased, the risk of cement fracture increased. This may be due to shifting the load-bearing support underneath the cement from cortical bone to trabecular bone.

A biomechanical analysis of initial fixation options for porous-tantalum-backed glenoid components

BACKGROUND

Porous-tantalum (PT)-backed glenoid components have recently been developed to improve fixation and minimize the incidence of glenoid component loosening, which remains a key limiting factor in long-term survival in total shoulder arthroplasty. PT-backed glenoids promote bony ingrowth as a method of preventing glenoid loosening at the prosthesis-glenoid interface. The use of polymethyl-methacrylate (PMMA) cement for initial fixation may prevent osteointegration due to mechanical occlusion of the porous surface and the nonosteoconductive properties of PMMA. This study aims to investigate alternative fixation methods of PT-backed glenoids in a biomechanical investigation.

MATERIALS AND METHODS

Nine PT-backed monoblock glenoid components were implanted in a polyurethane bone substitute using either press-fit, PMMA cement, or calcium phosphate cement techniques. A control group of 3 all-polyethylene pegged glenoid components was implanted with PMMA. Glenoid and humeral head components were fixed to a biomechanical testing machine for testing according to ASTM Standard F-2028. The humeral head was translated ±1.5 mm along the superior-inferior axis for 50,000 cycles for characterization of glenoid rocking and inferior-superior translation.

RESULTS

Glenoid compression and glenoid distraction followed similar patterns for PT-backed glenoids. Overall, the all-polyethylene cemented glenoid demonstrated superior fixation compared to all PT-backed groups throughout the test. Glenoids fixed with PMMA cement displayed more favorable initial fixation and resistance to glenoid motion throughout cyclic testing.

CONCLUSION

This study showed that among PT-backed glenoids, PMMA fixation provided an increase in stability during initial and final cycles compared to press-fit and calcium-phosphate fixation techniques. This improved stability may enhance the osteointegration of the implant.

Glenoid articular conformity affects stress distributions in total shoulder arthroplasty

BACKGROUND

The stress applied to the glenoid component in total shoulder arthroplasty (TSA) remains an important concern because of the risk of wear and loosening. The purpose of this study was to determine the stress pattern in the glenoid component with 3 different surface designs.

METHODS

Computer models of 9 scapulae of patients scheduled for TSA were created from computerized tomography images. Each glenoid was virtually reamed, and 3 different glenoid component designs (conforming, nonconforming, and hybrid) were placed. Using finite element analysis, superior translation of the humeral head was modeled. Maximum stress and shear stress were measured at 3 different locations in the glenoid component: center, transition, and superior regions.

RESULTS

All 3 designs showed a similar level of maximum stress at the center and transition regions, while the maximum stress at the superior periphery was significantly higher in the conforming design than in the other 2 designs (P = .0017). The conforming design showed significantly higher shear stress at the superior periphery (P < .0001).

DISCUSSION

Stress from periphery loading is higher than from the center and transition region regardless of component design and is highest in the conforming design. The stress at the transition region of the hybrid design was not higher than the other 2 designs. The hybrid design has favorable characteristics based on its low stress at the periphery and greater contact area with the humeral head at the center.

LEVEL OF EVIDENCE

Basic Science Study, Biomechanical Computer Simulation Study.

Fixation and durability of a bone-ingrowth component for glenoid bone loss

BACKGROUND

Deficient glenoid bone is a reconstructive challenge in shoulder arthroplasty. One solution is an ingrowth anatomic glenoid with column and screw fixation, with or without supplemental bone graft. This study examines the outcome of patients managed in this manner.

MATERIALS AND METHODS

This type of glenoid component was used in 21 shoulder arthroplasties with central or peripheral glenoid bone deficiencies: 13 for bone loss due to arthritic wear and 8 for revision arthroplasty. Patients were monitored clinically for a mean of 11.1 years (range, 7.6-15.1 years) and by x-ray imaging for a mean of 9.1 years (range, 2.2-14.2 years).

RESULTS

Revision procedures were needed for 7 shoulders at a mean of 10.4 years (range 5.5-14.3 years), 6 for polyethylene or metal wear leading to glenoid loosening in 4. In the 14 nonrevised shoulders, pain ratings (1 to 5 scale) decreased from a mean of 4.5 to 1.9 (P < .001). Mean active elevation increased from 100° to 125° (P = .02). Mean external rotation increased from 28° to 43° (P = .06). Results assessed by the Neer rating were excellent in 3, satisfactory in 10, and unsatisfactory in 1. In radiographic assessment of the unrevised shoulders, 4 were at risk for glenoid loosening, and 1 was at risk for humeral loosening.

CONCLUSIONS

This method of reconstruction can offer pain relief and improved motion. However, the large number of revision procedures and additional adverse changes on x-ray imaging suggest other reconstructive options may be more successful and durable.

Pressurisation leads to better cement penetration into the glenoid bone: a cadaveric study

The aim of this study was to compare a third-generation cementing procedure for glenoid components with a new technique for cement pressurisation. In 20 pairs of scapulae, 20 keeled and 20 pegged glenoid components were implanted using either a third-generation cementing technique (group 1) or a new pressuriser (group 2). Cement penetration was measured by three-dimensional (3D) analysis of micro-CT scans. The mean 3D depth of penetration of the cement was significantly greater in group 2 (p < 0.001). The mean thickness of the cement mantle for keeled glenoids was 2.50 mm (2.0 to 3.3) in group 1 and 5.18 mm (4.4 to 6.1) in group 2, and for pegged glenoids it was 1.72 mm (0.9 to 2.3) in group 1 and 5.63 mm (3.6 to 6.4) in group 2. A cement mantle < 2 mm was detected less frequently in group 2 (p < 0.001). Using the cement pressuriser the proportion of cement mantles < 2 mm was significantly reduced compared with the third-generation cementing technique.

Importance of polyethylene thickness in total shoulder arthroplasty: a finite element analysis

BACKGROUND

Articular surfaces reconstruction is essential in total shoulder arthroplasty. Because of the limited glenoid bone support, thin glenoid component could improve anatomical reconstruction, but adverse mechanical effects might appear.

METHODS

With a numerical musculoskeletal shoulder model, we analysed and compared three values of thickness of a typical all-polyethylene glenoid component: 2, 4 (reference) and 6mm. A loaded movement of abduction in the scapular plane was simulated. We evaluated the humeral head translation, the muscle moment arms, the joint force, the articular contact pattern, and the polyethylene and cement stress. Findings Decreasing polyethylene thickness from 6 to 2mm slightly increased humeral head translation and muscle moment arms. This induced a small decreased of the joint reaction force, but important increase of stress within the polyethylene and the cement mantel. Interpretation The reference thickness of 4mm seems a good compromise to avoid stress concentration and joint stuffing.

Interface micromotions increase with less-conforming cementless glenoid components

BACKGROUND

The optimal degree of conformity between the glenoid and humeral components in total shoulder arthroplasty for best performance and durability is still a matter of debate. The main aim of this study is to evaluate the influence of joint conformity on the bone-implant interface micromotions in a cementless glenoid implant.

MATERIALS AND METHODS

Polyethylene inlays with different degrees of conformity (radial mismatch of 0, 2, 4, and 6 mm) were mounted on a cementless metal back and then implanted in a bone substitute. These glenoid components were loaded by a prosthetic humeral head during a force-controlled experiment. Normal-to-interface micromotions and bone substitute deformations were measured at different points of the interface. Rim displacement and humeral head translation were also measured. A finite element (FE) model of the experiments was implemented to estimate the normal- and tangent-to-interface micromotions in the entire bone-implant interface.

RESULTS

All measured variables increased with less-conforming PE inlays. Normal-to-interface micromotions were significantly larger (P < .05) when the radial mismatch was 6 mm compared with the fully conforming inlay. The FE model was in agreement and complemented the experimental results. FE model-predicted interface micromotions were already significantly larger when the radial mismatch was equal to 4 mm.

DISCUSSION

In a force-controlled experiment with a cementless glenoid component, a non-conforming PE inlay allows larger interface micromotions than a conforming inlay, reaching a magnitude that may hamper local bone ingrowth in this type of component. This is mainly because of the larger humeral head translation that boosts the effects of the so-called rocking-horse phenomenon.

Effect of a pathological scapular tilt after total shoulder arthroplasty

Total shoulder arthroplasty (TSA) is an accepted and most successfully used treatment for different shoulder pathologies. Different risk factors for the failure of the prosthesis are known. A pathological scapular orientation, observed in elderly people or in patients suffering from neuromuscular diseases, could be a cause of failure, which has not been investigated yet. To test this hypothesis, a numerical musculoskeletal model of the glenohumeral joint was used to compare two TSA cases: a reference normal case and a case with a pathological anterior tilt of the scapula. An active abduction of 150° was simulated. Joint force, contact pattern, polyethylene and cement stress were evaluated for both cases. The pathological tilt slightly increased the joint force and the contact pressure, but also shifted the contact pattern. This eccentric contact increased the stress level within the polyethylene of the glenoid component and within the surrounding cement layer. This adverse effect occurred mainly during the first 60° of abduction. Therefore, a pathological orientation of the scapula may increase the risk of a failure of the cement layer around the glenoid component. These preliminary numerical results should be confirmed by a clinical study.

Thermal effects of glenoid reaming during shoulder arthroplasty in vivo

BACKGROUND

Glenoid component loosening is a common cause of failure of total shoulder arthroplasty. It has been proposed that the heat generated during glenoid preparation may reach temperatures capable of producing osteonecrosis at the bone-implant interface. We hypothesized that temperatures sufficient to induce thermal necrosis can be produced with routine drilling and reaming during glenoid preparation for shoulder arthroplasty in vivo. Furthermore, we hypothesized that irrigation of the glenoid during reaming can reduce this temperature increase.

METHODS

Real-time, high-definition, infrared thermal video imaging was used to determine the temperatures produced by drilling and reaming during glenoid preparation in ten consecutive patients undergoing total shoulder arthroplasty. The maximum temperature and the duration of temperatures greater than the established thresholds for thermal necrosis were documented. The first five arthroplasties were performed without irrigation and were compared with the second five arthroplasties, in which continuous bulb irrigation was used during drilling and reaming. A one-dimensional finite element model was developed to estimate the depth of penetration of critical temperatures into the bone of the glenoid on the basis of recorded surface temperatures.

RESULTS

Our first hypothesis was supported by the recording of maximum surface temperatures above the 56°C threshold during reaming in four of the five arthroplasties done without irrigation and during drilling in two of the five arthroplasties without irrigation. The estimated depth of penetration of the critical temperature (56°C) to produce instantaneous osteonecrosis was beyond 1 mm (range, 1.97 to 5.12 mm) in four of these patients during reaming and one of these patients during drilling, and two had estimated temperatures above 56°C at 3 mm. Our second hypothesis was supported by the observation that, in the group receiving irrigation, the temperature exceeded the critical threshold in only one specimen during reaming and in two during drilling. The estimated depth of penetration for the critical temperature (56°C) did not reach a depth of 1 mm in any of these patients (range, 0.07 to 0.19 mm).

CONCLUSIONS

Temperatures sufficient to induce thermal necrosis of glenoid bone can be generated by glenoid preparation in shoulder arthroplasty in vivo. Frequent irrigation may be effective in preventing temperatures from reaching the threshold for bone necrosis during glenoid preparation.

Polymethylmethacrylate: properties and contemporary uses in orthopaedics

Polymethylmethacrylate (PMMA) has been used in orthopaedics since the 1940s. Despite the development and popularity of new biomaterials, PMMA remains popular. Although its basic components remain the same, small proprietary and environmental changes create variations in its properties. PMMA can serve as a spacer and as a delivery vehicle for antibiotics, and it can be placed to eliminate dead space. Endogenous and exogenous variables that affect its performance include component variables, air, temperature, and handling and mixing. PMMA is used in hip arthroplasty and vertebral augmentation, notably, vertebroplasty and kyphoplasty. Cardiopulmonary complications have been reported.

Failure mechanism of the all-polyethylene glenoid implant

Fixation failure of glenoid components is the main cause of unsuccessful total shoulder arthroplasties. The characteristics of these failures are still not well understood, hence, attempts at improving the implant fixation are somewhat blind and the failure rate remains high. This lack of understanding is largely due to the fundamental problem that direct observations of failure are impossible as the fixation is inherently embedded within the bone. Twenty custom made implants, reflecting various common fixation designs, and a specimen set-up was prepared to enable direct observation of failure when the specimens were exposed to cyclic superior loads during laboratory experiments. Finite element analyses of the laboratory tests were also carried out to explain the observed failure scenarios. All implants, irrespective of the particular fixation design, failed at the implant-cement interface and failure initiated at the inferior part of the component fixation. Finite element analyses indicated that this failure scenario was caused by a weak and brittle implant-cement interface and tensile stresses in the inferior region possibly worsened by a stress raiser effect at the inferior rim. The results of this study indicate that glenoid failure can be delayed or prevented by improving the implant/cement interface strength. Also any design features that reduce the geometrical stress raiser and the inferior tensile stresses in general should delay implant loosening.

The glenoid in shoulder arthroplasty

Total shoulder arthroplasty is a common treatment for glenohumeral arthritis. One of the most common failure modes of total shoulder arthroplasty is glenoid loosening, causing postoperative pain, limitation of function, and potentially, the need for revision surgery. The literature has devoted considerable attention to the design of the glenoid component; efforts to better understand the biomechanics of the reconstructed glenohumeral joint and identify factors that contribute to glenoid component loosening are ongoing. This article reviews the current state of knowledge about the glenoid in total shoulder arthroplasty, summarizing the anatomic parameters of the intact glenoid, variations in component design and fixation, the mechanisms of glenoid loosening, the outcomes of revision surgery in the treatment of glenoid component failure, and alternative treatments for younger patients.

Numerical modelling of the shoulder for clinical applications

Research activity involving numerical models of the shoulder is dramatically increasing, driven by growing rates of injury and the need to better understand shoulder joint pathologies to develop therapeutic strategies. Based on the type of clinical question they can address, existing models can be broadly categorized into three groups: (i) rigid body models that can simulate kinematics, collisions between entities or wrapping of the muscles over the bones, and which have been used to investigate joint kinematics and ergonomics, and are often coupled with (ii) muscle force estimation techniques, consisting mainly of optimization methods and electromyography-driven models, to simulate muscular action and joint reaction forces to address issues in joint stability, muscular rehabilitation or muscle transfer, and (iii) deformable models that account for stress-strain distributions in the component structures to study articular degeneration, implant failure or muscle/tendon/bone integrity. The state of the art in numerical modelling of the shoulder is reviewed, and the advantages, limitations and potential clinical applications of these modelling approaches are critically discussed. This review concentrates primarily on muscle force estimation modelling, with emphasis on a novel muscle recruitment paradigm, compared with traditionally applied optimization methods. Finally, the necessary benchmarks for validating shoulder models, the emerging technologies that will enable further advances and the future challenges in the field are described.

Evaluation of the mechanical and architectural properties of glenoid bone

Successful glenoid fixation in shoulder arthroplasty is partly dependent on the properties of the underlying bone. Therefore, mapping of the glenoid surface and locating the bone with the highest quality, in terms of mechanical properties and morphology, is a key requirement in ensuring effective fixation. To this end, an investigation was undertaken to study the relationship between indentation behavior and the quality of the glenoid bone. Nineteen embalmed glenoids were obtained from human cadavers (mean age at death, 82 years). Each specimen was tested using a cylindrical indentor at 11 predetermined points to investigate load-displacement behavior. Microcomputed tomography analysis was performed to ascertain the bone volume (BV)/total volume (TV) fraction of the trabecular bone and the subchondral thickness. Statistical analysis showed that both strength and modulus varied with indentation position. Significant relationships were found between either strength or modulus and BV/TV or subchondral thickness, although the explained variance was relatively low.

Developments in shoulder arthroplasty

Indications for shoulder arthroplasty are numerous, mainly owing to glenohumeral osteoarthritis, rheumatoid arthritis, or fracture of the proximal humerus. However, the anatomy and the biomechanics of the shoulder are complex and shoulder arthroplasty has evolved significantly over the past 30 years. This paper presents the main recent evolutions in shoulder replacement, the questions not answered yet, and the main future areas of research. The review focuses firstly on the design, positioning, and fixation of the humeral component, secondly on the design, positioning, and fixation of the glenoid implant, and thirdly on other concepts of shoulder arthroplasty such as the reversed prosthesis, the cementless surface replacement arthroplasty, and the bipolar arthroplasty. This review demonstrates that more research is needed. Although, in the long term, large randomized trials are needed to settle the fundamental questions of what type of replacement and which kind of fixation should be used, biomechanical research in the laboratory should be focused primarily on the comprehension of glenoid loosening, which is a major cause of total shoulder arthroplasty failure, and the significance of radiolucent lines which are often seen but with no clear understanding about their relation with failure.

Risks of loosening of a prosthetic glenoid implanted in retroversion

Osteoarthritis of the shoulder is frequently associated with posterior glenoid wear, which may be difficult to correct during shoulder arthroplasty. This study was designed to evaluate the risks that a prosthetic glenoid implanted in retroversion will loosen. The scapula, the humerus, the rotator cuff, and a total shoulder prosthesis were reconstructed with a 3-dimensional finite element model. The glenoid was placed in 5 different angles of retroversion (0 degrees , 5 degrees , 10 degrees , 15 degrees , and 20 degrees ). Location of the glenohumeral contact point, articular pressure, bone and cement stress, and micromotion around the glenoid implant were calculated during internal and external rotation. Glenoid retroversion induced a posterior displacement of the glenohumeral contact point during internal and external rotation, inducing a significant increase of stress within the cement mantel (+326%) and within the glenoid bone (+162%). Furthermore, a major increase of micromotion was measured at the bone-cement interface (+706%). According to this study, glenoid retroversion exceeding 10 degrees should be corrected during total shoulder arthroplasty. If the correction is impossible, not replacing the glenoid should be considered.