For purposes of data analysis, the shoulders were divided into four groups based on glenohumeral prosthetic mismatch. Group 1 had a radial mismatch of ≥4 mm; group 2, a mismatch of 4.5 to 5.5 mm; group 3, a mismatch of 6 to 7 mm; and group 4, a mismatch of >7 to 10 mm. Demographic data were compared statistically among the groups.
Preoperative and postoperative information on each shoulder was collected with use of a standardized form. Clinical evaluation was performed with use of the absolute Constant score, which consists of four individual scores for pain (15 points), activity (20 points), active mobility (40 points), and strength (the number of kilograms of downwardly directed force that the patient is able to resist with the shoulder in 90° elevation in the plane of the scapula as measured with a dynamometer and multiplied by two) 25. Additionally, active forward elevation in the plane of the scapula and active external rotation with the arm at the side were recorded.
Anteroposterior radiographs with the humerus in neutral, internal, and external rotation and an axillary radiograph made at the time of the latest follow-up were evaluated for humeral and/or glenoid periprosthetic radiolucent lines and/or component loosening and proximal migration of the humeral component. The operating surgeon or a member of the surgical team classified any glenoid radiolucent lines according to the previously validated system of Molé et al., which assigns scores ranging from 0 points for no radiolucency to 18 points for radiolucent lines exceeding 2 mm in six zones ( Fig. 1 ) 23,26. A numeric value determined by the thickness of the radiolucent line is assigned to each zone: 0 points is given for no radiolucent line; 1 point, for a radiolucent line of <1 mm in thickness; 2 points, for a radiolucent line of 1 to 2 mm in thickness; and 3 points, for a radiolucent line of >2 mm in thickness. The scores for all of the zones are then added to yield the radiolucency score. A score of 0 to 6 points is considered to represent no loosening, 7 to 12 points represents possible loosening, and 13 to 18 points represents definite loosening. Complications or subsequent reoperations were also noted.
Statistically, the data were analyzed with use of the Fisher exact test and a Pearson chi-square test for qualitative variables. When quantitative values were evaluated, analysis of variance was performed to determine significance. Additionally, because of the possibility of the concomitant presence of multiple factors, an analysis was performed with use of a linear regression for quantitative dependant variables. A multivariate analysis and a Student t test were utilized to evaluate any relationship between quantitative and qualitative variables. To analyze the relationship between mismatch and radiolucency score, we performed a linear regression analysis and a variance analysis with one factor with linear and quadratic trend test. Significance was set at p < 0.05.
The duration of follow-up averaged 53.5 months (range, twenty-four to 110 months). Statistical analysis of the demographic data demonstrated no significant differences between the groups regarding gender, hand dominance, status of the rotator cuff at the time of surgery, age at the time of surgery, or duration of follow-up ( Table III ). Additionally, the experience of the surgeon did not influence the degree of mismatch that was selected.
Table IV shows the results of the clinical evaluation for each of the mismatch groups. With the numbers available, no significant difference was found between groups with regard to the Constant score, any component of the Constant score (pain, activity, mobility, or strength), or active forward elevation. Mismatch between 4.5 and 7 mm was associated with better active external rotation with the arm at the side (p = 0.001). With the numbers available, mismatch was not found to influence the rate or type of postoperative complications (anterior, posterior, or superior instability).
Radiographically, 243 of the 319 shoulders had a well-fixed glenoid component (a radiolucency score of <7 points), forty-nine shoulders had a glenoid component that was possibly loose (a radiolucency score of 7 to 12 points), and twenty-seven shoulders demonstrated loosening of the glenoid component (a radiolucency score of >12 points). Overall, the radiolucency score averaged 4.7 points (range, 0 to 18 ± 4.8 points).
The effect of glenohumeral mismatch on the radiolucency score was found to be highly significant (linear trend; p < 0.0001) as shown in Figure 2 and the scatterplot in Figure 3 . Table V shows the radiolucency scores for each category of mismatch. Group 1 was significantly different from group 3 (p < 0.002) and group 4 (p < 0.008). Group 2 was not found to be significantly different from the other groups with the numbers available.
Statistical analysis was performed to evaluate the potential influence of other factors (experience of the surgeon, presence of a partial or full-thickness tear of the rotator cuff, eccentric glenoid morphology, and occurrence of a postoperative complication) on the relationship between mismatch and the radiolucency score. The experience of the surgeon—that is, whether the surgeon had contributed ten cases or more to this series or had contributed less than ten cases—did not influence this relationship. Similarly, preoperative glenoid morphology, the occurrence of a postoperative complication, and the presence of a supraspinatus tear did not affect the relationship between mismatch and the radiolucency score.
This multicenter investigation demonstrated the relationship between glenoid component radiolucency and glenohumeral prosthetic mismatch. Despite the aforementioned in vitro investigations of glenohumeral mismatch 14-16, to our knowledge this in vivo relationship has not been described previously. The results of our investigation indicate that prosthetic glenohumeral radial mismatch in excess of 5.5 mm but no more than 10 mm most favorably influences the glenoid radiolucency score for the Aequalis prosthesis. Although a linear trend was discovered, the desired upper limit of radial mismatch has not been conclusively determined at this time since data obtained in this investigation are not amenable to extrapolation.
The importance of radiolucent lines adjacent to a glenoid component is unclear. Although some authors have reported that such lines have no influence on results or the revision rate, other investigators have found an association between the lines and compromised outcomes 27,28. Interestingly, in the present series, despite the relationship between glenohumeral mismatch and the formation of radiolucent lines, the mismatch had minimal effect on clinical results or complication rates. The only clinical parameter that it affected was postoperative external rotation, and the reason for this remains unclear. Although our findings appear to add more confusion to the issue of the importance of glenoid radiolucent lines, the lack of clinical influence may be explained by the relatively short-term follow-up of the patients in this study. In a long-term follow-up investigation, Torchia et al. found a relationship between compromised clinical results and glenoid radiolucent lines 28.
The current investigation is not without limitations. Multicenter study designs have inherent flaws. Different surgeons have different levels of experience, which can influence outcome. Additionally, many conditions (diagnosis, status of the rotator cuff, and glenoid morphology) can introduce confounding variables into an analysis of the outcomes of shoulder arthroplasty. To minimize the influence of these factors, we limited this series to patients with primary glenohumeral osteoarthritis, using strict exclusion criteria, even though that limited the extrapolation of our data to other diagnoses. Additionally, the influence of other confounding variables (surgeon experience, status of the rotator cuff, and glenoid morphology) was analyzed statistically to further validate the findings of this investigation.
Other factors limiting application of the data obtained in this study are the influence of prosthetic mismatch on glenohumeral stability and our inability to evaluate in vivo polyethylene wear. Joint conformity has been proposed as a factor affecting stability of the glenohumeral joint 29,30, and several investigators have studied this effect in a cadaveric model 9,11,13,31, with measurement of the minimum force necessary for joint dislocation. Severt et al. 11 and Fukuda et al. 31 concluded that greater joint conformity leads to higher subluxation forces. Karduna et al. showed that variation of radial mismatch between 0 and 5 mm changed dislocation forces by an average of only 3% 18. They concluded that these small differences were not clinically relevant. In our series of 319 total shoulder arthroplasties, only two patients had anterior instability; both were in the 4.5 to 5.5-mm-mismatch group. One patient, with ≤4 mm of mismatch, had posterior instability. Similarly, we evaluated the influence of mismatch and superior instability of the humeral head as characterized by a decrease in the acromiohumeral distance. Although eight shoulders had a decreased acromiohumeral distance at the time of follow-up, this factor had no significant relationship with prosthetic mismatch. In summary, after an average of fifty-four months of follow-up, we did not observe an influence of mismatch on anterior, posterior, or superior prosthetic stability.
A greater degree of glenohumeral mismatch allows more translation and a higher potential for polyethylene wear. To our knowledge, there is no reliable technique to evaluate wear of the polyethylene component quantitatively in total shoulder arthroplasty. Until it is possible to establish the influence of increased glenohumeral mismatch on polyethylene wear and its importance, the findings of this study must be applied with a degree of caution.
Despite these acknowledged imperfections, we believe that our study is the first in vivo investigation of the effects of glenohumeral prosthetic mismatch on clinical and radiographic parameters. On the basis of the results of this investigation, it appears that prosthetic radial mismatch in excess of 5.5 mm but no more than 10 mm most favorably influences the glenoid radiolucency score. Although a linear trend was discovered, the desired upper limit of radial mismatch was not conclusively determined since the data obtained in this investigation are not amenable to extrapolation. It is our hope that this investigation will be the first step in establishing a scientific basis for recommendations regarding glenohumeral prosthetic mismatch.
Investigation performed at the Department of Orthopaedic Surgery, Clinique Sainte Anne Lumière, Lyon, France
In support of their research or preparation of this manuscript, one or more of the authors received grants or outside funding from Association pour le Développement de la Pathologie de l'Epaule. In addition, one or more of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commer- cial entity (Tornier Company). Also, a commercial entity (Tornier Company) paid or directed, or agreed to pay or direct, benefits to a research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.
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