Intrinsic stability of unused and retrieved polyethylene glenoid components

Inclusion of Glenoid Anteversion Provides a More Accurate Assessment of Glenoid Stability Using a Measuring Protocol for the Modified Bony Shoulder Stability Ratio

PURPOSE

To clarify whether there is a disparity between the conventional bony shoulder stability ratio (cBSSR) calculated using the method of Moroder et al. and the stability ratio (SR) obtained biomechanically and whether the modified bony shoulder stability ratio (mBSSR) calculated using the modified method, adjusted for glenoid anteversion, shows good consistency with the biomechanically determined SR.

METHODS

Forty-two glenoid models were successively constructed from seven cadaveric scapular bones, each with varying degrees of bone defect (intact condition and 2-, 4-, 6-, 8-, and 10-mm defects). The cBSSR and mBSSR were calculated using the conventional and modified radiologic protocols, respectively. A biomechanical experiment was conducted to measure the biomechanical SR of the glenoid model for accuracy validation. Linear regression analysis, intraclass correlation coefficient (ICC) calculation, Bland-Altman plot generation, and repeated-measures analysis of variance were performed to compare these methods to ascertain the impact of including glenoid anteversion on the accuracy of the bony shoulder stability ratio (BSSR).

RESULTS

The mBSSR, which included glenoid anteversion, showed a stronger correlation with the biomechanical SR compared with the cBSSR. Linear regression analysis showed R2 = 0.7727 and ICC = 0.726 for the mBSSR versus the biomechanical SR and showed R2 = 0.5507 and ICC = 0.363 for the cBSSR versus the biomechanical SR. Bland-Altman analysis revealed less bias between the mBSSR and biomechanical SR (bias, 0.0854; 95% confidence interval, -0.0762 to 0.2470) than between the cBSSR and biomechanical SR (bias, 0.1899; 95% confidence interval, 0.0039 to 0.3759). Repeated-measures analysis of variance confirmed a significant difference between the cBSSR and biomechanical SR (P = .002).

CONCLUSIONS

The inclusion of glenoid anteversion in mBSSR calculations provides a more accurate assessment of glenoid stability. Our findings indicate the need to consider anteversion adjustments in BSSR estimation.

CLINICAL RELEVANCE

Our research identified that conventional methods did not take glenoid anteversion into account. Through our comprehensive biomechanical experiment, we have shown that incorporating glenoid anteversion in the BSSR calculation yields a more precise assessment of glenoid stability, which can provide a crucial methodologic foundation for clinical assessment.

The ream and run: not for every patient, every surgeon or every problem

PURPOSE

The purpose of this paper is to provide some essential and basic information concerning the ream and run technique for shoulder arthroplasty.

METHODS

In a total shoulder arthroplasty, the humeral head prosthesis articulates with a polyethylene glenoid surface placed on the bone of the glenoid. Failure of the glenoid component is recognised as the principal cause of failure of total shoulder arthroplasty. By contrast, in the ream and run procedure, the humeral head prosthesis articulates directly with the glenoid, which has been conservatively reamed to provide a stabilising concavity and maximal glenohumeral contact area for load transfer. While no interpositional material is placed on the surface of the glenoid, animal studies have demonstrated that the reamed glenoid bone forms fibrocartilage, which is firmly fixed to the reamed bony surface. Glenohumeral motion is instituted on the day of surgery and continued daily after surgery to mold the regenerating glenoid fibrocartilage. When the healing process is complete - as indicated by a good and comfortable range of motion - exercises and activities are added progressively without concern for glenoid component failure. 

RESULTS

The experience to date indicates that a technically well done ream and run procedure can restore high levels of comfort and function to carefully selected patients with osteoarthritis, capsulorrhaphy arthroplathy, and posttraumatic arthritis.

CONCLUSIONS

Patients considering the ream and run procedure should understand that this technique avoids the risks and limitations associated with a polyethylene glenoid component, but that it requires strong motivation to follow through on a rehabilitation course that may require many months. The outcome of this procedure depends on the body's regeneration of a new surface for the glenoid and requires rigorous adherence to a daily exercise program. This paper explains in detail the principal factors in patient selection and the key technical elements of the procedure. Clinical examples and outcomes are demonstrated.

Automated In-Vitro Testing of Orthopaedic Implants: A Case Study in Shoulder Joint Replacement

This investigation presents the design and preliminary validation of a single station simulator with biaxial motion and loading designed to mimic the kinematics of the glenohumeral joint during arm abduction in the scapular plane. Although the design of the glenoid holder allows the glenoid component to translate in all three axes, it is primarily loaded axially, which brings it into contact with the oscillating humeral head, but is also loaded superiorly to simulate common subluxation of the humeral head. Simulating arm abduction in the scapular plane simplifies component alignment and removes the need for anterior—posterior loading, thereby creating a stable joint without the need to simulate capsular constraints. In this more physiologically accurate simulator design, the load and motion profiles influence the contact kinematics, but the wear path is ultimately determined by the conformity and constraint designed into the bearing couple. The wear data are determined and correlated with clinically retrieved glenoid components, as well as previously reported in-vitro studies, thus verifying use of the simulator in testing alternative materials and designs. The key design features, as well as the improvements proposed through this study, can be incorporated into the design of test fixtures for any other orthopaedic implant such as the hip, knee, spine, elbow, and finger.

Glenohumeral contact kinematics in patients after total shoulder arthroplasty

BACKGROUND

Knowledge of in vivo glenohumeral joint contact mechanics after total shoulder arthroplasty may provide insight for the improvement of patient function, implant longevity, and surgical technique. The objective of this study was to determine the in vivo glenohumeral joint contact locations in patients after total shoulder arthroplasty. We hypothesized that the glenohumeral joint articular contact would be centered on the glenoid surface because of the ball-in-socket geometric features of the implants.

METHODS

Dual-plane fluoroscopic images and computer-aided design models were used to quantify patient-specific glenohumeral articular contact in thirteen shoulders following total shoulder arthroplasty. The reconstructed shoulder was imaged at arm positions of 0 degrees, 45 degrees, and 90 degrees of abduction (in the coronal plane) and neutral rotation and at 90 degrees of abduction with maximum internal and external rotation. The patients were individually investigated, and their glenohumeral joint contact centroids were reported with use of contact frequency.

RESULTS

In all positions, the glenohumeral joint contact centroids were not found at the center of the glenoid surface but at an average distance (and standard deviation) of 11.0 +/- 4.3 mm from the glenoid center. Forty (62%) of the sixty-five total contact occurrences were found on the superior-posterior quadrant of the glenoid surface. The position of 0 degrees of abduction in neutral rotation exhibited the greatest variation of quadrant contact location; however, no contact was found on the superior-anterior quadrant of the glenoid surface in this position.

CONCLUSIONS

In vivo, glenohumeral joint contact after total shoulder arthroplasty is not centered on the glenoid surface, suggesting that kinematics after shoulder arthroplasty may not be governed by ball-in-socket mechanics as traditionally thought. Although contact locations as a function of arm position vary among patients, the superior-posterior quadrant seems to experience the most articular contact in the shoulder positions tested.

In-vitro evaluation of a polyurethane compliant-layer glenoid for use in shoulder arthroplasty

A polyurethane glenoid component has been designed and manufactured as part of a total shoulder arthroplasty (TSA) system based on compliant-layer (CL) technology. Compared with conventional TSA designs, this biomimetic approach offers reduced friction and wear and potentially improved longevity. In-vitro evaluation of the glenoid system has included loosening and stability tests, and wear measurement using a specially constructed wear simulator. The results obtained support the hypothesis that a CL glenoid design may provide improved resistance to dynamic loosening and rim erosion, and demonstrate superior wear performance over a standard ultra-high molecular weight polyethylene design. This study not only confirms the feasibility of a CL glenoid component but also highlights the potential to increase implant longevity, thereby allowing earlier surgical intervention before poor glenoid bone stock and soft tissue compromise the outcome of TSA.

Improved accuracy of computer assisted glenoid implantation in total shoulder arthroplasty: an in-vitro randomized controlled trial

BACKGROUND

Glenoid replacement is challenging due to the difficult joint exposure and visualization of anatomical reference landmarks. Improper positioning of the glenoid component or inadequate correction of the retroversion using currently available instrumentation may lead to early failure. The objective of this study was to evaluate a computer-assisted technique to achieve a more accurate placement of the glenoid component compared to traditional techniques.

METHODS

Sixteen paired cadaveric shoulders were randomized to either traditional or computer-assisted glenoid implantation. Preoperative planning consisting of CT scanning with 3-dimensional image modeling of the shoulder specimens and intraoperative tracking with real-time feedback provided to the surgeon was employed in the computer-assisted group. A validated, previously published, standardized protocol for tracking the orientation of the glenoid in space using 3 glenoid surface landmarks was employed. All phases of glenoid implantation (initial guide pin insertion, reaming, drilling of the peg holes, and final component implantation) were tracked and recorded by the computer. A post-implantation CT scan was performed in both groups to compare how accurately the implants were placed.

RESULTS

The computer-assisted technique was more accurate in achieving the correct version during all phases of glenoid implantation and as measured on the post-implantation CT scan (P < .05). The largest errors with traditional glenoid implantation were observed during drilling and, more so, during reaming. The trend was to overly retrovert the glenoid.

CONCLUSIONS

Computer assisted navigation results in a more accurate glenoid component placement relative to traditional techniques.

LEVEL OF EVIDENCE

Basic Science Study.

Glenoid component failure in total shoulder arthroplasty

Glenoid component failure is the most common complication of total shoulder arthroplasty. Glenoid components fail as a result of their inability to replicate essential properties of the normal glenoid articular surface to achieve durable fixation to the underlying bone, to withstand repeated eccentric loads and glenohumeral translation, and to resist wear and deformation. The possibility of glenoid component failure should be considered whenever a total shoulder arthroplasty has an unsatisfactory result. High-quality radiographs made in the plane of the scapula and in the axillary projection are usually sufficient to evaluate the status of the glenoid component. Failures of prosthetic glenoid arthroplasty can be understood in terms of failure of the component itself, failure of seating, failure of fixation, failure of the glenoid bone, and failure to effectively manage eccentric loading. An understanding of these modes of failure leads to strategies to minimize complications related to prosthetic glenoid arthroplasty.

Shoulder arthroplasty: the socket perspective

Although much attention has been directed to the development of the humeral components used in shoulder arthroplasty, the major unsolved challenge lies on the glenoid side of the articulation. This challenge arises from difficulties resisting eccentric loading and providing adequate implant-bone fixation. Current glenoid component designs use polyethylene and polymethyl methacrylate and are prone to loosening, plastic deformation, particulate debris, and third-body wear. Metal-backed components present further challenges, and results have generally been disappointing. There is interest in biologic resurfacing procedures, including the interposition of fascia, capsule, or meniscal allograft and nonprosthetic glenoid arthroplasty, or what has become known as the "ream-and-run" procedure. Despite encouraging results, important questions remain unanswered about these procedures. However, each may warrant further exploration with a goal of providing an effective and durable approach to glenoid arthritis that avoids the risks associated with polymethyl methacrylate and polyethylene.

Direct injection of blood into the labrum enhances the stability provided by the glenoid labral socket

We tested the hypothesis that the stabilizing function of the labrum can be enhanced by inflating it with blood. In 6 fresh cadaveric glenoids, the anteroinferior stability provided by the glenoid was quantitated by measuring the maximal angle between the glenoid centerline and the direction of the force applied via a ball in the glenoid before the ball dislocated from the glenoid. This stability angle was measured for each of 4 different applied loads. These measurements were repeated after the anteroinferior labrum was augmented by the injection of fresh blood. Injection augmentation of the labrum significantly increased the measured stability angles in 5 of 6 specimens. The 1 outlier had a partial labral tear. The mean increase in stability for all 6 glenoids ranged from 19% to 30% for the different test loads. Labral injection with blood may be a useful adjunct in the surgical management of glenohumeral instability.

Alterations in surface geometry in retrieved polyethylene glenoid component

This study characterized and quantified the changes found in retrieved glenoid polyethylene components found at revision total shoulder arthroplasty (TSA). Twenty components obtained at revision TSA were evaluated, all from a system (Global, DePuy, Warsaw, IN) with a glenoid radius of curvature 3 mm greater than that of the humeral head. Laser surface scanning provided three-dimensional analysis of the surface of the glenoid component. Scans of unused components of similar sizes enabled determination of the changes occurring after implantation. Alterations in radius of curvature were noted in every glenoid. All showed loss of the balance stability angle (BSA, the maximal angle that the net humeral joint reaction force can make with the glenoid center line before the humeral head would dislocate) of at least 5 degrees in one or more directions. Increase in BSA in one direction was seen in 11 of the components. In five of these, the increase was associated with a reduction of the local radius to match that of the humeral component. Glenoid surface morphology and stability can be changed by in vivo use. While correlation with clinical instability in the patients from whom the implants were obtained was not possible, many of the observed changes in surface morphology are of sufficient magnitude to compromise the contribution of the glenoid surface to shoulder stability. Three patterns of wear were identified: "humeral" that showed loss of the mismatch between the humeral and glenoid radii of curvature (5 of 20 components), "diffuse" that showed broad surface irregularity (18 of 20), and "rim" wear with loss of the polyethylene rim of the component (14 of 20). More than one type of wear was possible within a single glenoid.

Capsulolabral augmentation by blood injection increases the intrinsic stability provided by the glenoid

PURPOSE

To test the hypothesis that the intrinsic stability of the cadaveric glenoid can be augmented by the injection of blood into the labrum.

METHODS

The intrinsic stability of 10 fresh frozen cadaveric glenoids was assessed by measuring the balance stability angle. Pretreatment values of 8 directions in each glenoid were obtained. The labrum was then injected with blood freshly drawn from a volunteer. After the injected blood clotted, measurements of the balance stability angle of the 8 directions were again obtained.

RESULTS

The mean pre-injection balance stability angle for 7 of the 8 directions were significantly increased by the injection of blood: anterosuperior, from 25 to 35 degrees (p<0.005); anterior, from 27 to 34 degrees (p<0.01); anteroinferior, from 36 to 39 degrees (p<0.005); inferior, from 38 to 41 degrees (p<0.02); posteroinferior, from 35 to 42 degrees (p<0.01); posterior, from 27 to 35 degrees (p<0.0005); and posterosuperior, from 26 to 29 degrees (p<0.005). Cross-sections of injected labra demonstrated a firm clot within the labrum with substantial increases in thickness.

CONCLUSION

The intrinsic stability of cadaveric glenoids can be significantly augmented by the injection of blood into the labrum. It is possible that blood injection may be a useful primary or adjunct procedure in the open or arthroscopic management of glenohumeral instability.

Healing of reamed glenoid bone articulating with a metal humeral hemiarthroplasty: a canine model

This study characterizes the healing response of the glenoid after spherical reaming and prosthetic humeral head replacement in a canine model of glenohumeral hemiarthroplasty. The right glenoid of twelve skeletally mature female dogs was reamed to a uniform radius of curvature, removing all cartilage down to bleeding subchondral bone. The glenoid was not resurfaced. The humeral head was replaced with a stemmed metal prosthesis. Post-surgery, the operated limbs were immobilized for seven days, with motion allowed ad libitum thereafter. Fluorescent bone labels were administered to identify bone formation. These procedures were not complicated by instability, infection or death. Six animals were euthanized at 10 week and six more at 24 week. The intact glenohumeral joints were evaluated by gross examination, assessment of glenoid concavity, and light microscopy of methylmethacrylate sections. At 10 week, vascular fibrous tissue partially covered the glenoid, maintaining a concave surface congruent with the prosthetic humeral head. New bone formed at the margin of the glenoid, and the density of the periarticular trabecular bone increased. At 24 week, the healing was more advanced; thick fibrocartilaginous tissue covered the entire glenoid surface. These results demonstrate that spherical glenoid reaming produced a consistent healing response characterized by remodelling of the reamed bony concavity to a congruent, living, smooth, securely attached interface articulating with the humeral prosthesis.

Use of three-dimensional computed tomography for the analysis of the glenoid anatomy

Preoperative evaluation for a total shoulder arthroplasty includes 2-dimensional analysis of the glenoid through either standard radiographs or computed tomography (CT) images. Recent evidence suggests that these 2-dimensional images may actually misrepresent the 3-dimensional (3D) anatomy of the glenoid. Because 3D reconstructions of CT images allow 3D visualization and analysis of the scapula as a free body, we hypothesized that they can reflect the true anatomy of the glenoid more accurately. To test this hypothesis, we obtained various glenoid morphometric measurements from excised cadaveric scapulae as well as their respective 3D CT images. On average, the glenoid version angles measured from the 3D CT images were within 1.0 degrees +/- 0.7 degrees (mean +/- SD) of those from the actual specimen (95% confidence limit, <2.2 degrees for all observers). These measurements from the 3D CT images showed high interobserver and intraobserver reliability (interobserver and intraobserver correlation coefficients, 0.983 and 0.978, respectively). Similarly, measured glenoid surface width and length from the 3D CT images were within 1.8 +/- 1.2 mm and 1.4 +/- 1.1 mm, respectively, of those from the actual specimen. In addition, we were able to estimate the glenoid surface area as well as the glenoid vault volume from the 3D CT images. These values were 8.67 +/- 2.73 cm2 and 11.86 +/- 5.06 cm3, respectively. The mean glenoid vault volume with respect to its surface area was 1.35 +/- 0.24 cm3/cm2 (range, 1.06-1.91 cm3/cm2). These data suggest that 3D CT images can accurately reflect the true anatomy of the glenoid and that they can provide valuable information regarding the glenoid surface and vault. As such, 3D CT images may prove to be a useful tool during the preoperative evaluation for a total shoulder arthroplasty, particularly in patients with significant glenoid bone loss.

Optimizing the glenoid contribution to the stability of a humeral hemiarthroplasty without a prosthetic glenoid

BACKGROUND

In a shoulder requiring arthroplasty, if the glenoid is flat or biconcave, the surgeon can restore the desired glenoid stability by using a glenoid prosthesis with a known surface geometry or by modifying the surface of the glenoid to a geometry that provides the desired glenoid stability. This study tested the hypotheses that (1) the stability provided by the glenoid is reduced by the removal of the articular cartilage; (2) the stability contributed by the glenoid is compromised by loss of its articular cartilage, and this lost stability can be restored by spherical reaming along the glenoid centerline; and (3) the stability of a reamed glenoid is comparable with that of a native glenoid and with that of a polyethylene glenoid with similar surface geometry; and (4) the glenoid stability can be predicted from the glenoid surface geometry.

METHODS

The stability provided by the glenoid in a given direction can be characterized by the maximal angle that the humeral joint reaction force can make with the glenoid centerline before the humeral head dislocates; this quantity is defined as the balance stability angle in the specified direction. The balance stability angles were both calculated and measured in eight different directions for an unused polyethylene glenoid component and eleven cadaveric glenoids in four different states: (1) native without the capsule or the rotator cuff, (2) denuded of cartilage and labrum, (3) after reaming the glenoid surface around the glenoid centerline with use of a spherical reamer with a radius of 25 mm, and (4) after reaming around the glenoid centerline with use of a spherical reamer with a radius of 22.5 mm.

RESULTS

The calculated and measured balance stability angles for each direction in each glenoid were strongly correlated. Denuding the glenoids of the articular cartilage reduced the glenoid contribution to stability, especially in the posterior direction. Reaming the glenoid restored the stability to values comparable with those of the normal glenoid. For example, the average calculated balance stability angle (and standard deviation) in the posterior direction for all eleven glenoids was 24 degrees for the native glenoids, 14 degrees for the denuded glenoids, 25 degrees for the glenoids reamed to a radius of 25 mm, and 33 degrees for the glenoids reamed to a radius of 22.5 mm. The values for the glenoids reamed to 25 mm (25 degrees ) were similar to those of a polyethylene glenoid of the same radius of curvature. For glenoids reamed to 22.5 mm, the average difference between the actual balance stability angle and that predicted from the glenoid geometry was 3.4 degrees +/- 2.4 degrees.

CONCLUSIONS

The glenoid contribution to shoulder stability was decreased by the removal of cartilage and labrum and was restored by spherical reaming to a level similar to resurfacing the glenoid with a polyethylene component.

Observations on retrieved glenoid components

To obtain more information on the pattern of damage of prosthetic glenoid components, we analyzed 7 retrieved glenoid components. The consecutive series included 2 standard polyethylene components and 5 highly crystalline polyethylene glenoids (Hylamer; DePuy Dupont Orthopaedics, Warsaw, IN) retrieved 3 to 12 years after implantation. At revision, 4 of 5 Hylamer components were fractured. Common wear patterns were i) deformation and crumbling of the rim, particularly at the inferior hemicircumference, probably caused by direct contact of the humerus with the prosthetic component; ii) roughening (abrasion and scratching) of the adjacent articulating surface; and iii) concentric and congruous wear centered posteriorly. Available glenoid components may cover an excessive sector of the head. This can result in mechanical restriction of glenohumeral motion and abutment of the humerus against the glenoid rim. Abutment may cause major shear forces and therefore cause glenoid loosening. The value of articular surface mismatch is questionable because retrieved glenoids were worn to a conforming joint.