Abstract
Background: Considerable interest has been focused on the design of the glenoid component used in total shoulder arthroplasty in order to reduce the risk of loosening. One design-related feature that has attracted attention is whether to use pegged or keeled cemented glenoid components. The main purpose of this study was to compare the fixation of cemented keeled glenoid components with that of cemented in-line pegged glenoid components.
Methods: In a prospective randomized study, we compared the stability of cemented, all-polyethylene, keeled glenoid components and cemented, all-polyethylene, in-line three-pegged glenoid components by radiostereometric analysis. Twenty-seven shoulders in twenty-five patients with osteoarthritis (twenty-two shoulders had primary and five shoulders had secondary osteoarthritis) were included. There were sixteen women and nine men, and the mean age was sixty-four years. Radiostereometric analysis and conventional radiographs were carried out at five days, at four months, and at one and two years postoperatively.
Results: The mean Constant and Murley score preoperatively and two years postoperatively was 25 and 70, respectively, for shoulders with the keeled glenoid component and 22 and 70 for the shoulders with a pegged component. No significant difference was detected between groups with regard to the average micromigration of the glenoid components at any of the time points. The average translation was <1 mm, while the median value was <0.3 mm at two years, with no significant difference between the different axes. In five shoulders (three with the keeled component and two with the pegged component), translation at two years was >1 mm. In fourteen shoulders (eight with the keeled and six with the pegged component), the rotation around one or several axes was >2°. We were not able to detect any specific pattern with regard to movement for either type of component nor were we able to detect any difference between the two types of components in the way they migrated, if migration occurred.
Conclusions: Cemented all-polyethylene keeled or in-line three-pegged glenoid components appear to have similar stability during the first two years after surgery. Studies with a longer follow-up period are needed to relate these findings to long-term clinical and radiographic outcomes.
Level of Evidence: Therapeutic Level I. See Instructions to Authors for a complete description of levels of evidence.
In shoulder arthroplasty, there is controversy as to whether the glenoid should be resurfaced1,2. One reason for this debate is related to the common occurrence of radiolucent lines and the risk that such radiolucencies might indicate subsequent loosening of the glenoid component. In hip and knee replacement, a gradual development of radiolucent lines over time seems to correspond with clinical loosening3. When evaluating radiolucent lines around the glenoid component, there is often the problem that such lines can be seen on the immediate postoperative radiographs4. Valid interpretation of follow-up radiographs is therefore very difficult. In a previous study with a two-year follow-up with use of radiostereometric analysis to measure micromotion, we showed that glenoid components were often well-fixed even in the presence of radiolucent lines on conventional radiographs5.
One question with regard to glenoid design that has attracted a great deal of attention is whether to use pegged or keeled components. Biomechanical studies6, animal studies7, retrospective studies4, and one prospective radiographic study by Gartsman et al.8 have indicated that cemented pegged glenoid components have less apparent loosening than cemented keeled glenoid components. In a recent radiostereometric study by Nuttall et al.9, pegged components were found to be more stable than keeled glenoid components, indicating better fixation for the pegged components.
The main purpose of this study was to compare the fixation of cemented keeled glenoid components with that of in-line pegged glenoid components by measuring micromotion with use of radiostereometry. Our hypothesis was that pegged components would be more stable than keeled glenoid components in total shoulder arthroplasty.
Between 2001 and 2004, we prospectively included twenty-eight consecutive patients (thirty shoulders) with osteoarthritis, all of whom were scheduled for total shoulder replacement. All patients had given their informed consent prior to inclusion. The study was approved by the ethical committee of Uppsala University Hospital. This clinical trial was registered with the Ethical Committee Register of Uppsala University (number Ups 01-079). Three patients (three shoulders, including two with the keeled component and one with the pegged component) were excluded from the study as the radiostereometric analysis software showed instability of the tantalum markers in the glenoid socket, which made it impossible to perform the radiostereometric examinations; this left twenty-seven shoulders in twenty-five patients for the study. One patient with a keeled component died from unrelated causes prior to the two-year follow-up, leaving twenty-six shoulders (twelve with the keeled and fourteen with the pegged component) for the final follow-up evaluation at two years. There were sixteen women and nine men, with a mean age of sixty-four years (range, forty-two to eighty-one years). Two women had bilateral replacements, which had been done on different occasions. The diagnosis was primary osteoarthritis in twenty-two shoulders and secondary osteoarthritis in five shoulders. The secondary arthritis was due to osteonecrosis in three, fracture in one, and repeated dislocations in one.
Two experienced shoulder surgeons performed the operations using the Bigliani/Flatow (H.R. and J.N.) total shoulder prosthesis (Zimmer, Warsaw, Indiana). The humeral stem was inserted without cement with use of a press-fit technique in all but one shoulder, in which it was cemented. An offset humeral head was used in all but two shoulders in order to make it possible to achieve better coverage of the humeral osteotomy. All patients had an intact rotator cuff. One patient had the outer end of the clavicle resected, and eight patients had a tenodesis of the long head of the biceps tendon.
During surgery, the patients were allocated by block randomization, with use of a closed envelope technique, to receive either a keeled (thirteen shoulders) or a three-pegged, all-polyethylene glenoid component marked with five tantalum markers with a diameter of 0.8 mm (fourteen shoulders). The markers were positioned in the glenoid implant through five small drill-holes made in the polyethylene. The technique has been used extensively in different hip and knee implant studies and is considered as a standardized technique for implantation of markers when performing radiostereometric studies3,5,9-13. The stability of each marker is calculated by a specific feature in the radiostereometric analysis software, i.e., any potential loose or unstable marker can be identified and excluded when assessing micromotion.
The glenoid component has a convex back and both a conforming and nonconforming (mismatch) zone of articulation in relation to the radius of the prosthetic humeral head. The conforming zone is in the center of the glenoid, and the radius of the articular surface of the glenoid matches the radius of the humeral head. On the basis of theoretical assessments, this permits maximum glenoid contact and theoretically lower stresses in the polyethylene when the head is centered. At the periphery, the radius of the articular surface of the glenoid is greater than the radius of the humeral head by at least 14 mm, which allows translation of the head to reduce the potential so-called rocking-horse effect caused by translation. The glenoid components were inserted with use of Palacos R cement with gentamicin (Schering-Plough, Malmö, Sweden) in all shoulders.
During surgery, five tantalum markers with a diameter of 1.0 mm were injected into the glenoid socket, one in the coracoid process, and two in the acromion, with a custom-made delivery device, before the glenoid component was implanted. The spheres were spaced as widely as possible to optimize the subsequent radiostereometric evaluation.
The radiostereometric examinations were done with the patient sitting in front of the x-ray cassettes on a chair, with the arm hanging along the side of the trunk. The two x-ray tubes that allowed simultaneous exposure were placed at about 60° to each other in front of the patient, with a horizontal beam direction. Exact positioning of the tubes was not critical, since the three-dimensional coordinates of each x-ray tube were calculated at each examination. The film-to-focus distance was about 100 cm. The first radiostereometric examination (reference) was done five to seven days after surgery. The following examinations were done after four months and at one and two years after surgery.
The radiostereometric technique allows three-dimensional, high-resolution assessment of motion between segments defined by the tantalum markers. The technique has been described extensively and has been used by preference for the assessment of the migration of hip and knee prostheses10-12. Our radiostereometric measurements were done with the UmRSA system (RSA Biomedical, Umeå, Sweden). The movements of the glenoid component in relation to the glenoid socket were recorded with use of the markers in the glenoid, acromion, and coracoid process as the fixed reference segment. The linear movements were expressed as translations along the transverse axis (x axis; medial [+] — lateral [—] translation), the longitudinal axis (y axis; proximal [+] — distal [—] translation), and the sagittal axis (z axis; anterior [+] — posterior [—] translation). The angular movements were expressed as rotations around the transverse axis (x axis; forward [+] — backward [—] rotation), the longitudinal axis (y axis; retroversion [+] — anteversion [—]), and the sagittal axis (z axis; valgus [+] — varus [—]), respectively (Figs. 1-A, 1-B, and 1-C). The evaluation performed a few days after surgery was used as the reference examination. All movement is described as the movement that had occurred compared with the position on the first examination.
At four months, the precision of the method was determined by repeated examinations in all patients, with an interval of less than five minutes between the examinations. The absolute mean value of the recorded differences between the two repeated examinations ± 2.7 standard deviations represented the 99% confidence limit13. Thus, the levels for a minimally detectable significant migration (p < 0.05) for translations were >0.10 mm (transverse axis), >0.12 mm (longitudinal axis), and >0.19 mm (sagittal axis). The corresponding levels for significant rotations were >0.93° (transverse axis), >1.13° (longitudinal axis), and >1.29° (sagittal axis). Migration values below these levels were classified as not significant, i.e., no detectable migration in the individual shoulder.
Conventional anteroposterior and lateral radiographs were made postoperatively and at one and two years. The first postoperative anteroposterior radiograph was compared with those made at one and two years, with respect to radiolucencies, by an experienced radiologist (L.W.), who was blinded with respect to the clinical result and the radiostereometric results. Radiolucencies around the glenoid components were graded between 0 and 5, according to the criteria described by Gartsman et al.8, with 0 representing no lucency and 5 indicating gross loosening of the component.
The clinical results were assessed with use of the Constant and Murley score14 before surgery and at each follow-up evaluation. Patients also made a subjective assessment of their own shoulder function to determine the subjective shoulder value, which is expressed as a percentage of what they considered to be normal shoulder function15. The subjective shoulder value was recently validated and compared with the Constant and Murley score16. The subjective shoulder value was assessed by the patients before surgery and one and two years thereafter.
Statistical Methods
An unpaired Student t test was performed to evaluate the significance of differences in micromotion, Constant and Murley score, and subjective shoulder value between shoulders treated with a keeled component and those treated with a pegged component. An analysis of variance test of repeated measures was performed to evaluate the significance of differences in Constant and Murley scores and subjective shoulder values within each treatment group over time. The Fisher exact test was used to evaluate the significance of differences in the proportion of radiolucent lines. A power analysis was done to estimate the sample size. A total of twenty-four subjects, twelve in each group, was assumed to be adequate in order to detect a significant difference between the two groups, with a = 0.05 and ß = 0.20 (i.e., a power of 0.8). The analysis was performed on the assumption that one group would have an average translation of 0.5 mm. Our goal was to detect an increase in the average translation of 0.25 mm. The input data for this assessment were taken from a previous study by us on glenoid components5. On the basis of that study, we powered the study to be able to detect a difference in the primary variable, i.e., translation, between groups of 0.25 mm. No power analysis was made on the secondary variables, i.e., findings on conventional radiographs and clinical findings. A p value of <0.05 was considered significant.
Source of Funding
The work was supported in part with a research grant from Zimmer to the institution where the work was performed as stated in the disclosure. There was no other potential conflict of interest for any of the authors.
Demographic Data
The age, sex, preoperative Constant and Murley score, and subjective shoulder value for the two treatment groups are listed in Table I.
Clinical Findings
The Constant and Murley score improved significantly (p < 0.001) from a mean of 25 points (range, 1 to 54 points) before surgery to a mean of 70 points (range, 29 to 95 points) at the two-year follow-up evaluation, when all shoulders were included. The corresponding scores for the treatment groups were 25 points preoperatively, 70 points at one year, and 70 points at two years for the shoulders with a keeled glenoid component and 22, 67, and 70 points, respectively, for the shoulders with a pegged component. No significant difference in these scores between the groups was detected at any time point. The average subjective rating by the patients of the shoulder function (described as the percentage of that of the normal shoulder) was 37% before surgery and 84% at both the one-year and two-year follow-up evaluations, with all twenty-seven shoulders included (p < 0.0001). The corresponding subjective rating, i.e., the subjective shoulder value, for the two groups was 35 preoperatively, 81 at one year, and 80 at two years for the shoulders with the keeled glenoid component and 39, 81, and 80, respectively, for the shoulders with a pegged glenoid component. No significant difference between the treatment groups was detected at any of the time points. A prestudy power analysis was only done for micromigration and not for radiolucencies or clinical variables. The lack of a significant difference can therefore have been due to too few subjects for these variables.
Two shoulders had a reoperation during the follow-up period: one because of anterior instability and the other because of posterior instability. Both shoulders were included in the study since the glenoid components (one keeled and one pegged) were found to be well-fixed during the reoperation.
Radiographic Results
On the immediate postoperative radiographs, one shoulder with a keeled component had grade-1 radiolucency, while the remaining twenty-six shoulders had no radiolucency8. At two years, nine of the twelve shoulders with a keeled glenoid component had lucency, while eight of fourteen shoulders with a pegged component had lucency (p = 0.429). No component had grade-4 or grade-5 lucency.
Radiostereometry
The linear movements, i.e., the translations, and the rotational movements between the glenoid components and the fixed bone segment in the scapula are given in Tables II and III. No significant difference (power, 0.82) between the groups with respect to the average movement of the components was detected at any time point. The average translation for all three axes and for both groups was <1 mm, while the median value was <0.3 mm at two years, with no significant difference noted among the different axes.
In five shoulders (three with a keeled component and two with a pegged component), translation at two years was >1 mm. For the three shoulders with a keeled glenoid component with translation of >1 mm, the maximal direction of movement was 3.15 mm of lateral translation in one (Case 22), while the other two had medial translation of 1.01 mm (Case 18) and 1.47 mm (Case 26). The maximal translation for the two shoulders with a pegged glenoid component was 2.65 mm of lateral translation in one (Case 4) and 8.91 mm of posterior translation in the second (Case 13). For one patient (Case 13), the gross migration shown on radiostereometric analysis corresponded to signs of implant migration and/or loosening on the conventional radiographs. For the other glenoid components, the conventional radiographs did not reveal any evidence of migration or loosening.
In fourteen shoulders (eight with a keeled component and six with a pegged component), the rotation around one or several axes was >2°. For the eight shoulders with a keeled component with rotation of >2°, three rotated around the z axis: two into valgus angulation by 4.48° (Case 19) and 12.48° (Case 26), respectively, while the third rotated into varus angulation by 2.67° (Case 21). For two components, rotation was most pronounced around the x axis, with both rotating backward by 2.38° (Case 15) and 10.39° (Case 22). For three components, rotation was most pronounced around the y axis, with all moving in anteversion by 5.58° (Case 18), 3.99° (Case 24), and 5.23° (Case 25). For the pegged components, the maximal angulation occurred as a valgus movement of the glenoid in four shoulders, with 2.13° (Case 3), 3.51° (Case 1), 7.10° (Case 8), and 7.42° (Case 4) of angulation at two years. The remaining two components with angulation of >2° rotated around the x axis: 3.57° backward in one patient (Case 11) and 4.83° forward in one patient (Case 13).
On the basis of the components with pronounced movement mentioned above, and for the components with movement of >1 mm and 2°, we were unable to detect any specific pattern with regard to movement for either of the two component types. We also did not detect any specific difference between the two types of components in the way they migrated, if migration occurred.
Radiolucent Lines and Micromigration
In all, nine glenoids had no radiolucent lines (grade 0) at the final follow-up evaluation, while six were judged as having grade-1 radiolucency and the remaining eleven were judged as having grade-2 or 3 radiolucency. Glenoids with grade-2 or 3 radiolucency tended to have a greater degree of micromotion along, and around, all three axes compared with glenoids without radiolucent lines (grade 0) or those with grade 1. The observed differences in micromotion for the different grades of radiolucency were not significant (analysis of variance with Bonferroni post hoc test). A prestudy power analysis was only done for micromigration and not for correlation of radiolucencies and micromigration. The lack of a significant difference can therefore have been due to too few subjects for this assessment.
There has been considerable interest in the design of the glenoid component used in total shoulder arthroplasty. The main reason for this is the high prevalence of radiolucent lines and concerns about glenoid loosening. One design-related question that has attracted attention is whether to use pegged or keeled cemented glenoid components. In a biomechanical study, Roche et al.17 found no difference in edge displacement after dynamic eccentric loading. Each keeled or pegged glenoid component remained firmly fixed after testing. In a three-dimensional finite analysis, Lacroix et al.6 showed that a pegged glenoid component was superior for normal bone, whereas a keeled component was better for bone in patients with rheumatoid arthritis. On the other hand, in another finite element analysis, Mansat et al.18 reported that eccentric loading caused cracks in the cement mantle, although no significant difference was seen between keeled and pegged implants. It is important to emphasize that those studies were biomechanical or theoretical analyses and were not based on the outcome of clinical studies.
In the present study, we used radiostereometric analysis to compare the fixation of cemented keeled glenoid components with that of cemented pegged glenoid components in patients with osteoarthritis. The radiostereometric technique has a very high accuracy, and it can therefore be used to detect even very small amounts of implant migration, so-called micromigration. The technique has been widely used to study hip and knee implants and their fixation10-12, but we know of only four previous studies in which it was used to measure the fixation of cemented glenoid implants5,9,19,20. In the present study, which compared keeled and pegged glenoid components, no significant difference in micromovements was found between the designs. In a recently published radiostereometric study that is very similar to the present one, Nuttall et al.9 concluded that both keeled and pegged components moved, but the keeled components migrated significantly more, especially in patients with glenoid erosions. They used the Global Shoulder Arthroplasty System (DePuy International, Leeds, England) in which the glenoid component has five pegs placed in a configuration that is different from that in the Bigliani/Flatow system (Zimmer) used in the present study. Mechanical tests have shown that the peg design has an influence on the shear stability of implants21. The explanation for the lack of difference in micromotion between the pegged and keeled components in the present study might be that, in the Bigliani/Flatow concept, three pegs are placed in a row, making the pegged glenoid resemble a keeled component (Fig. 1-A). In the study by Nuttall et al.9, they found a distinctive pattern of migration of the keeled component around the longitudinal axis. Their findings were in accordance with a previous study from our group of cemented, all-polyethylene keeled glenoid components. In that study, we found that most of the translation occurred in the distal direction and that angulation was most pronounced around the longitudinal axis in retroversion or anteversion5. In the present study, we did not find this type of specific pattern of migration for either the keeled or the pegged components.
The quality and the amount of the glenoid bone present at the time of the arthroplasty are important for the fixation of the glenoid component. Nuttall et al.9 made preoperative assessments of the glenoid bone stock on plain radiographs or computed tomography scans and classified them according to the method of Walch et al.22. We did not use computed tomography scans routinely in this series, and found it difficult to estimate the degree of glenoid erosion on plain radiographs. During surgery, we reamed the glenoid to obtain a congruent match between the subchondral plate and the back of the component. The lack of preoperative objective assessment of the quality of the glenoid bone with a more precise method than plain radiographs is, of course, a limitation of this study. As the group of patients was fairly homogeneous and the randomization seems to have provided comparative groups on the basis of known demographics, it seems reasonable to assume that randomization provided similar groups with respect to the distribution of bone quality although this could not be shown because of a lack of objective bone stock measurements.
In another radiostereometric study, Nuttall et al.19 reported that pegged glenoid components in combination with an offset humeral head had less micromotion than when combined with a non-offset head. They speculated that offset heads may reduce eccentric loading. In the present study, we used offset heads in all but two shoulders.
Limitations of the present study include the small number of patients and the short follow-up period. The number of patients was based on a power analysis aimed to address the primary outcome variable, i.e., micromotion measured with radiostereometric analysis. On the basis of this power analysis, a total of twenty-four patients, twelve in each group, was considered adequate to address the primary variable. Because of experience from previous studies, it was anticipated that 5% to 10% of the patients would have to be excluded because of instability of the tantalum markers in the glenoid bone. Three patients had to be excluded for this reason. They were all early in the series. In all shoulders, marker instability occurred in the glenoid socket because of malpositioning of one or several markers, while all of the markers in the implant were stable. There was no suspicion that these patients had weaker bone or any other cause for the marker instability. The unstable tantalum markers were discovered by the software included in the radiostereometric system during the normal procedure to check marker stability. In most instances, so-called marker instability means movement of a marker of only a few micrometers, but that is enough to classify it as unstable as it will reduce the precision of the measurements. It is important to remember that instability of the markers implanted in the bone has nothing to do with instability of the glenoid implant.
Because of the high accuracy of radiostereometric analysis, the number of patients can be kept low and still allow reasonably strong conclusions to be drawn with regard to micromotion. In comparison, it is well known that research questions related to clinical outcome and findings based on conventional radiographs need larger numbers of patients to reach conclusive results. In the present study, clinical outcome variables and findings based on conventional radiographs were defined as secondary outcome variables. The lack of significant differences between the treatment groups with respect to these secondary variables could therefore have been due to the study being underpowered for those variables.
The short duration of follow-up is obviously a limitation. However, it has been shown for both knee and hip implants that the amount of micromotion detected by radiostereometric analysis during the first two years following surgery can be predictive of loosening several years later. For instance, tibial components with <0.2 mm of maximal migration between twelve and twenty-four months have been defined as stable with a low risk for loosening, while implants with migration of >0.2 mm have been defined as unstable and/or loose11. Because of a lack of radiostereometric studies with a longer follow-up interval after shoulder arthroplasty at this time, it is not possible to say whether the amount of early micromotion seen in glenoid components can be used to predict loosening. Studies with a longer follow-up period are needed to address this important question.
One major controversy with regard to total shoulder arthroplasty is how to interpret the frequently seen radiolucencies around cemented glenoid components. The question is whether such a radiographic sign indicates the presence of loosening, whether it predicts loosening, or whether there is a low correlation between limited radiolucency and mechanical loosening20. Two radiographic studies have compared the presence of radiolucent lines at the glenoid bone-cement interface with pegged and keeled glenoid components4,8. In both studies, pegged glenoid components had less lucency. In the present study, no significant relation was identified between radiolucencies and migration. The small number of patients and the short follow-up interval make it difficult to assess the importance of this finding.
In conclusion, we found that the glenoid components were well-fixed, with the exception of one component that was obviously loose. There was no difference in total migration and migration patterns between the two glenoid designs. Both components appear to have similar stability during the first two years after surgery. Studies with a longer follow-up period are needed to relate these findings to long-term clinical and radiographic outcomes.
Note: The authors thank Monica Gelotte for making the radiostereometric radiographs and Berit Svensson for measuring the Constant and Murley score.
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