Characteristics of unsatisfactory shoulder arthroplasties

Finite element modelling of glenohumeral kinematics following total shoulder arthroplasty

Due to the shallowness of the glenohumeral joint, a challenging but essential requirement of a glenohumeral prosthesis is the prevention of joint dislocation. Weak glenoid bone stock and frequent dysfunction of the rotator cuff, both of which are common with rheumatoid arthritis, make it particularly difficult to achieve this design goal. Although a variety of prosthetic designs are commercially available only a few experimental studies have investigated the kinematics and dislocation characteristics of design variations. Analytical or numerical methods, which are predictive and more cost-effective, are, apart from simple rigid-body analyses, non-existent. The current investigation presents the results of a finite element analysis of the kinematics of a total shoulder joint validated using recently published experimental data for the same prostheses. The finite element model determined the loading required to dislocate the humeral head, and the corresponding translations, to within 4% of the experimental data. The finite element method compared dramatically better to the experimental data (mean difference=2.9%) than did rigid-body predictions (mean difference=37%). The goal of this study was to develop an accurate method that in future studies can be used for further investigations of the effect of design parameters on dislocation, particularly in the case of a dysfunctional rotator cuff. Inherently, the method also evaluates the glenoid fixation stresses in the relatively weak glenoid bone stock. Hence, design characteristics can be simultaneously optimised against dislocation as well as glenoid loosening.

Deficits in shoulder function and general health associated with sixteen common shoulder diagnoses: a study of 2674 patients

The purpose of this study is to define the self-assessed deficits in function and general health perceived by a large cohort of patients with common shoulder diagnoses. For a 10-year period, all new shoulder patients presenting to the senior author were characterized by diagnosis, age, gender, and self-assessed shoulder function and general health status. This report concerns the 2674 patients having 1 of the 16 most prevalent diagnoses. The deficits in shoulder function and health status were correlated with diagnosis, age, and gender, as well as with each other. Patient self-assessment provided a standardized method for collecting data on shoulder function and health status. These assessments revealed substantial deficits; for example, 81% of patients were unable to sleep on the affected side, and 71% were unable to wash the back of the opposite shoulder. The magnitude of these deficits correlated significantly with gender and diagnosis but not with age.

Effect of joint conformity on glenoid component fixation in total shoulder arthroplasty

Results of shoulder replacements are inferior and must be improved. Two of the major problems of total shoulder replacements are loosening of cemented glenoid components and wear of polyethylene inlays of uncemented, metal-backed glenoid components. The aim of this study is to investigate the influence of joint conformity on glenoid-component fixation. Keeled glenoid components, with radii of curvature of 24, 25, or 29 mm, were cemented in bone substitutes, placed in a force-controlled test set-up, articulating against a 24 mm humeral head. They were loaded by a constant joint compression force (725 ± 10 N) and a superior subluxation force (shear force), cyclically varying between 0 and 350 ± 1 N. After 200 000 load cycles, the upper and lower glenoid component rim-displacements were measured by custom-made displacement sensors. Additionally, the shear-out strength has been measured to investigate the residual strength.

The glenoid component structures with radii of curvature of 24, 25, and 29 mm showed maximum superior rim-displacements of 0.163 (SD = 0.01), 0.299 (SD = 0.0306), and 0.350 (SD = 0.0197) mm respectively, which is a significant difference (p < 0.05). The maximum shear-out strength of glenoid components with radii of curvature of 24, 25, and 29 mm was 2707 (SD = 452), 2648 (SD = 299), and 2631 (SD = 312) N respectively, which is not a significant difference (p < 0.05). However, the results indicate that a conform articulation shows smaller glenoid rim-displacements, which might be beneficial for long-term component fixation.

Effect of Interface Conditions between Ultrahigh Molecular Weight Polyethylene and Polymethyl Methacrylate Bone Cement on the Mechanical Behaviour of Total Shoulder Arthroplasty

The aim of this study was to investigate the effect of the interface condition between polymethyl methacrylate (PMMA) bone cement and the ultrahigh molecular weight (UHMWPE) glenoid component on cement stresses and glenoid component tilting in a finite element (FE) model. The background of this research is that most FE models assume bonding between the PMMA bone cement and the UHMWPE component, although it is very doubtful that this bonding is present.

An FE model of a cemented glenoid component was developed and a joint compression force and subluxation force of 725 and 350 N respectively were applied. The maximal principal stresses in the cement layer ranged between 21.30 and 32.18 MPa. Glenoid component tilting ranged between 0.943° and 0.513°.

It was found that the interface condition has a large effect on the maximal principal stresses and glenoid component tilting. Whether adhesion between the UHMWPE component and PMMA bone cement occurs is unknown beforehand and, as a result, design validation using the FE technique should be carried out both by using contact elements in combination with a coefficient of friction as well as by a full bonding at this interface.

Characterizing the effect of diagnosis on presenting deficits and outcomes after total shoulder arthroplasty

This study compared self-assessed deficits in comfort, function, and health status before and after total shoulder arthroplasty for 4 different diagnoses: degenerative joint disease (DJD), secondary DJD (2 degrees DJD), rheumatoid arthritis (RA), and capsulorrhaphy arthropathy (CA). Deficits were assessed by the Simple Shoulder Test and Short Form 36 (SF-36) questionnaires. There was a significant difference among diagnoses for preoperative and postoperative functional deficits. The profiles of improvement within the categories of comfort, motion, strength, and function were different for each diagnosis. Patients with DJD and CA were most improved in the category of motion, whereas those with 2 degrees DJD and RA were most improved in the category of comfort. There was also a statistically significant difference in 5 of the 8 domains of the preoperative SF-36 among diagnoses. Factors associated with each diagnosis play a significant role in determining the magnitude of preoperative deficits and postoperative improvement in shoulder function.

Finite element models of total shoulder replacement: application of boundary conditions

The widespread use of FEA within orthopaedics is often prohibited by the limits of available computational power, with simplifications to the model often necessary in order to permit solution. An example of this includes the use of osseous models that exclude muscular loading, and may consist of only a partial or truncated region of the anatomy. However, is it possible to make such simplifications without affecting the predictive quality of the model? This issue has been considered using the specific example of the total shoulder reconstruction, where the effects of including the entire osseous region and/or the muscle loadings, has been evaluated. The effect of including the muscle loadings and the entire osseous structure was seen to increase with distance from the articular surface of the glenoid prosthesis. Stresses in the cement mantle were reduced in the absence of either the entire scapula bone or the muscle loading. The study suggests that the use of a fully defined scapula (hard- and soft-tissue) is particularly important when investigating fixation, whilst less comprehensive models should be appropriate for studies of the prosthesis exclusively.

Patient functional self-assessment in late glenoid component failure at three to eleven years after total shoulder arthroplasty

Failure of the glenoid component is the most common indication for late revision of a total shoulder arthroplasty (TSA). This is the first study to characterize the deterioration in patient self-assessment of shoulder function occurring with glenoid component failure at times remote from the index surgery. Of 115 total shoulders, 11 had revision by the original surgeon for isolated glenoid loosening. Simple Shoulder Test scores averaged 4.4 before TSA, rose to a mean of 11.3 after surgery, and fell to a mean of 4.6 before revision for glenoid loosening performed at a mean of 7 years after TSA. All shoulders showed a drop of at least 3 points between the peak Simple Shoulder Test score and the prerevision Simple Shoulder Test score. Periodic self-assessment of shoulder function may offer a method of screening patients for the possibility of late glenoid component failure.

Radiographic comparison of pegged and keeled glenoid components

Glenoid loosening is one reason for failure of total shoulder arthroplasty. Several factors, including radiographic lucency, have been shown to be associated with glenoid loosening. The purpose of this study was to assess the correlation between glenoid design and immediate radiographic lucency in a prospective randomized clinical trial. Total shoulder arthroplasty was performed in 43 patients over a 2-year period. Twenty-three patients were randomized into the keel group and twenty patients into the pegged group. Postoperative radiographs obtained within 6 weeks of surgery were evaluated by 3 raters to determine glenoid lucency. On a scale from 0 (no lucency) to 5 (gross lucency and component loosening), the rate of lucency was 39% (9/23) in the keeled components, which was significantly higher than the rate of 5% (1/20) observed in the pegged components (P = .026). Patient age, gender, and glenoid size did not significantly affect glenoid component lucency (P > .05). The consistency reliability among raters (Cronbach alpha) was 0.87, and the intertester reliability was 0.87. Pegged glenoid components have less radiographic lucency when compared with keeled glenoid components in the immediate postoperative period.

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.

Effect of glenoid component inclination on its fixation and humeral head subluxation in total shoulder arthroplasty

OBJECTIVE

To investigate the effect of glenoid component inclination angle on the fixation of cemented glenoid-bone structures and humeral head subluxation.

BACKGROUND

Results of shoulder replacements, in terms of glenoid component fixation and joint functionality, are bad and must be improved. Repeated glenoid component tilting, a result of eccentric contact forces harms glenoid component fixation. However, the effect of glenoid component inclination is unknown.

METHODS

Keeled glenoid components are cemented into bone substitutes and positioned under inclination angles varying between -4.5 degrees and +4.5 degrees . For each inclination angle 5 glenoid specimens are loaded by a horizontal, constant joint compression force (725 (SD 10) N) and a vertical, superior directed subluxation force (shear force), cyclically varying between 0 and 350 (SD 1) N. After 200,000 load cycles, the upper and lower glenoid component rim-displacements are measured during 1500 additional load cycles by custom made displacement sensors.

RESULTS

The maximal superior rim-displacement significantly increased and the minimal inferior rim-displacement dramatically decreased for increasing glenoid component inclination angles up to +4.5 degrees . Maximal ratio of subluxation force over compression force significantly decreased for increasing glenoid component inclination.

CONCLUSIONS

According to this study, decreasing glenoid component inclination angle decreases glenoid component tilting and humeral head subluxation and the results can be used to improve total shoulder replacements.

RELEVANCE

Better understanding of the effect of glenoid component inclination on glenoid component tilting and humeral head subluxation may be useful information for the surgeon to improve results of the total shoulder arthroplasty, when replacing the glenoid surface.

Effects of shoulder arthroplasty and exercise in patients with rheumatoid arthritis

The aim of this study was to examine pain and shoulder function in patients with rheumatoid arthritis (RA) before and after shoulder arthroplasty and postoperative exercise. Twenty-four patients (26 shoulders) were consecutively included in a multicentre study. Before surgery, at discharge from hospital and after 3 and 6 months, perceived shoulder function and shoulder pain were assessed by visual analogue scales, activities of daily living by the Modified Health Assessment Questionnaire (M-HAQ) and shoulder range of motion (ROM) by a goniometer. All patients showed considerable pain reduction at discharge from hospital (p<0.001). In those with intact rotator cuff and biceps tendon (n=13) improvements were found after 6 months in active and passive abduction and flexion ROMs (p<0.01) and in M-HAQ (p<0.001). Such improvements were not found in those with torn soft tissue (n=12). Preoperatively, abduction and flexion motor deficits (passive ROM >active ROM) were found for the total group (p=0.001). Less flexion motor deficit was found in the intact soft tissue than in the torn soft tissue group after 3 (p=0.002) and 6 months (p<0.001). No group difference was found with respect to abduction motor deficit. In conclusion, pain relief was obtained by all patients. Improvements in ROMs and activities of daily living were influenced by the state of the soft tissue.

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.

Preoperative factors associated with improvements in shoulder function after humeral hemiarthroplasty

BACKGROUND

The relationship between the characteristics of the shoulder that can be determined before humeral hemiarthroplasty and the functional improvement after surgery is not known. The goal of this study was to test the hypothesis that the functional outcome of this procedure correlated significantly with factors that are identifiable preoperatively.

METHODS

The study group included seventy-one shoulders in sixty-eight patients undergoing hemiarthroplasty, performed by the same surgeon, for diagnoses other than acute fracture. The mean age of the patients was sixty-one years (range, thirty to eighty-three years). The results were characterized in terms of the change in self-assessed shoulder function and general health status at an average of forty-nine months (range, twenty-four to 142 months) after surgery.

RESULTS

The preoperative absence of erosion of the glenoid was associated with greater improvement in shoulder function and level of comfort after hemiarthroplasty (p < 0.001). Shoulders that had not had previous surgery had greater functional improvement than did those that had previous surgery (p = 0.012). Shoulders with an intact rotator cuff showed significantly (p < 0.5) greater improvement in the ability to lift weight above shoulder level after hemiarthroplasty (p <0.5). With regard to diagnoses, shoulders with rheumatoid arthritis, capsulorrhaphy arthropathy, and cuff tear arthropathy had the least functional improvement, whereas those with osteonecrosis (p = 0.0004) and with primary (p = 0.02) and secondary degenerative joint disease (p = 0.03) had the greatest improvement. Patient age and gender did not significantly affect the outcome.

CONCLUSIONS

These results suggest that the functional improvement following humeral hemiarthroplasty is related to factors that are identifiable before surgery. These data may be of benefit in preoperative discussions with patients who have a shoulder disorder and are considering treatment with hemiarthroplasty.

Glenoid cementing may generate sufficient heat to endanger the surrounding bone

Glenoid loosening is a common complication of shoulder arthroplasty. One possible cause is bone necrosis from the exothermic reaction of polymethylmethacrylate. The relationship between the amount of cement used in glenoid fixation and the risk of thermal injury to bone was examined. Glenoid arthroplasty was done on 17 fresh cadaver scapulas, recording the amount of cement used. The bone surface temperature during cement curing was measured using infrared thermography. Using these data and published thresholds for thermal necrosis, the frontal plane area of bone that would be at risk for necrosis in vivo was estimated. The average weight of cement implanted was 5.35 g (2.65-8.08 g). The maximum temperature recorded averaged 64.7 degree C (48.2 degree-76.8 degree C). The area of bone at risk correlated with the amount of cement used. This study indicates that potentially dangerous amounts of heat may be generated during cementing of glenoid components.