Orthopedics

Feature Article 

Eccentric Reaming in Total Shoulder Arthroplasty: A Cadaveric Study

Robert Gillespie, MD; Robert Lyons, MD; Mark Lazarus, MD

Abstract

Posterior glenoid bone loss often is seen in association with glenohumeral osteoarthritis. Many different techniques have been proposed to account for this bone loss during total shoulder arthroplasty, the most popular being eccentric anterior reaming. However, the amount of correction that can be achieved has not been been well quantified. The purpose of this study was to define the amount of eccentric posterior glenoid wear that can be corrected by anterior glenoid reaming. Eight cadaveric scapulae were studied. Simulations of posterior glenoid wear in 5° increments were performed on each scapula. The specimens were then eccentrically reamed to correct the deformity. Anteroposterior width, superior-inferior height, and the best-fit pegged glenoid prosthesis size were measured. Anterior reaming to correct a 10° posterior defect resulted in a decrease in anteroposterior glenoid diameter from 26.7±2.5 mm to 23.8±3.1 mm (P=.006). In 4 of 8 specimens, placing a glenoid prosthesis was not possible after correcting a 15° deformity because of inadequate bony support (N=2), peg penetration (N=1) or both (N=1). A 20° deformity was correctable in 2 of 8 specimens and only after downsizing the glenoid component. Anterior glenoid reaming to correct eccentric posterior wear of >10° results in significant narrowing of the anteroposterior glenoid width. A 15° deformity has only a 50% chance of successful correction by anterior, eccentric reaming. Orthopedic surgeons need to be cognizant of this in their preoperative planning for total shoulder arthroplasty.

Glenohumeral osteoarthritis often exhibits a pattern of eccentric posterior glenoid erosion.1,2 This poses a difficult problem that must be corrected during total shoulder arthroplasty to prevent abnormal loading, instability, and poor clinical results.3-6 Various techniques have been proposed to correct excessive glenoid retroversion, including increased humeral anteversion, bone grafting, use of an augmented glenoid component, or eccentric reaming of the glenoid.7,8

Many authors have advocated bone grafting to correct severe glenoid deformities.9,10 Neer and Morrison11 recommended augmentation of the glenoid when instability can not be corrected by standard techniques, by either making minor changes in humeral version or contouring the glenoid subchondral bone. However no quantitative guidelines were given. Ibarra et al10 suggested that bone grafting or the use of a custom glenoid component may be necessary to correct deformities >20°, but no justification for this was provided. Friedman et al12 recommended bone grafting if the retroversion of the glenoid surface exceeded 15° as determined by computed tomography. However, bone grafting for any amount of eccentric glenoid wear is technically difficult and has been associated with variable clinical success,7,11,13-15 thus other options to correct posterior erosion during shoulder arthroplasty have been studied.

Techniques to correct excessive glenoid retroversion have also included increasing humeral anteversion.16,17 However, Spencer et al18 showed that anteverting the humeral component to compensate for posterior glenoid bone loss led to no increase in biomechanical stability. Rice et al19 have shown that an augmented glenoid component did not always lead to correction of instability.

The most common method for correction of posterior glenoid wear is with eccentric reaming and it has been suggested that mild posterior glenoid wear can be corrected by eccentric preferential reaming of the anterior (high) side.20,21 The primary goal of glenoid reaming is to obtain excellent congruent contact between the bone and the glenoid prosthesis to minimize any rocking.10,22 It is clear, however, that successfully accomplishing this task is difficult, even in expert hands. Lazarus et al23 radiographically studied 328 implanted glenoids and defined a grading scale for completeness of glenoid component seating. Fifteen (38.5%) of 39 keeled components and 85 (29.4%) of 289 pegged components had “poor seating” by their criteria.23 The most common cause for incomplete seating was incomplete…

Abstract

Posterior glenoid bone loss often is seen in association with glenohumeral osteoarthritis. Many different techniques have been proposed to account for this bone loss during total shoulder arthroplasty, the most popular being eccentric anterior reaming. However, the amount of correction that can be achieved has not been been well quantified. The purpose of this study was to define the amount of eccentric posterior glenoid wear that can be corrected by anterior glenoid reaming. Eight cadaveric scapulae were studied. Simulations of posterior glenoid wear in 5° increments were performed on each scapula. The specimens were then eccentrically reamed to correct the deformity. Anteroposterior width, superior-inferior height, and the best-fit pegged glenoid prosthesis size were measured. Anterior reaming to correct a 10° posterior defect resulted in a decrease in anteroposterior glenoid diameter from 26.7±2.5 mm to 23.8±3.1 mm (P=.006). In 4 of 8 specimens, placing a glenoid prosthesis was not possible after correcting a 15° deformity because of inadequate bony support (N=2), peg penetration (N=1) or both (N=1). A 20° deformity was correctable in 2 of 8 specimens and only after downsizing the glenoid component. Anterior glenoid reaming to correct eccentric posterior wear of >10° results in significant narrowing of the anteroposterior glenoid width. A 15° deformity has only a 50% chance of successful correction by anterior, eccentric reaming. Orthopedic surgeons need to be cognizant of this in their preoperative planning for total shoulder arthroplasty.

Glenohumeral osteoarthritis often exhibits a pattern of eccentric posterior glenoid erosion.1,2 This poses a difficult problem that must be corrected during total shoulder arthroplasty to prevent abnormal loading, instability, and poor clinical results.3-6 Various techniques have been proposed to correct excessive glenoid retroversion, including increased humeral anteversion, bone grafting, use of an augmented glenoid component, or eccentric reaming of the glenoid.7,8

Many authors have advocated bone grafting to correct severe glenoid deformities.9,10 Neer and Morrison11 recommended augmentation of the glenoid when instability can not be corrected by standard techniques, by either making minor changes in humeral version or contouring the glenoid subchondral bone. However no quantitative guidelines were given. Ibarra et al10 suggested that bone grafting or the use of a custom glenoid component may be necessary to correct deformities >20°, but no justification for this was provided. Friedman et al12 recommended bone grafting if the retroversion of the glenoid surface exceeded 15° as determined by computed tomography. However, bone grafting for any amount of eccentric glenoid wear is technically difficult and has been associated with variable clinical success,7,11,13-15 thus other options to correct posterior erosion during shoulder arthroplasty have been studied.

Techniques to correct excessive glenoid retroversion have also included increasing humeral anteversion.16,17 However, Spencer et al18 showed that anteverting the humeral component to compensate for posterior glenoid bone loss led to no increase in biomechanical stability. Rice et al19 have shown that an augmented glenoid component did not always lead to correction of instability.

The most common method for correction of posterior glenoid wear is with eccentric reaming and it has been suggested that mild posterior glenoid wear can be corrected by eccentric preferential reaming of the anterior (high) side.20,21 The primary goal of glenoid reaming is to obtain excellent congruent contact between the bone and the glenoid prosthesis to minimize any rocking.10,22 It is clear, however, that successfully accomplishing this task is difficult, even in expert hands. Lazarus et al23 radiographically studied 328 implanted glenoids and defined a grading scale for completeness of glenoid component seating. Fifteen (38.5%) of 39 keeled components and 85 (29.4%) of 289 pegged components had “poor seating” by their criteria.23 The most common cause for incomplete seating was incomplete correction of posterior wear by anterior reaming. However, the limitations of glenoid reaming can be seen when posterior glenoid wear is severe, as attempts to fully correct the glenoid version with reaming can result in excessive medialization of the glenoid component or inability to implant the glenoid secondary to inadequate bone stock.24

Recently, Clavert et al21 showed that retroversion greater than >15° could not be satisfactorily corrected by eccentric reaming but little else has been written quantifying the amount of glenoid wear amenable to eccentric reaming.

The purpose of this study was to define the amount of posterior glenoid wear that could be corrected by eccentric reaming in a cadaveric model. We hypothesized that there would be a threshold amount of eccentric posterior glenoid wear beyond which correction by anterior glenoid reaming would not be possible.

Materials and Methods

Eight unpaired human cadaver scapulae (4 left, 4 right) were studied. All soft tissue attachments including the glenoid labrum, glenohumeral capsular ligaments, and periscapular muscles were removed from the specimens. The scapulae were examined and none showed evidence of osteo-, rheumatoid-, post-traumatic arthritis, or congenital deformity and thus none were excluded.

Each specimen was placed in a vise and positioned using a standard level such that the glenoid articular surface was level and parallel to the floor. The largest anteroposterior (AP) width of the glenoid articular surface was measured and recorded using a caliper that was accurate to 0.1 mm. One half of the AP width (measured from the anterior edge) was then marked on the glenoid face using a longitudinal hashmark.

The largest superoinferior dimension of the glenoid articular surface was then measured. One half of the superoinferior distance (measured from the inferior edge) was marked on the glenoid face using a transverse hashmark. Thus the point where the 2 hashmarks intersected was the anatomic center of the glenoid face.

Using the Stryker Orthopaedics (Kalamazoo, Michigan) Solar Total Shoulder System, the best fit glenoid surface trial was recorded. This system had several features that made it ideal for this cadaver study. First, all reamer sizes are the same radius of curvature of 24 mm thus controlling for this variable. Glenoid component fit, therefore, could be solely judged by osseous dimensions. Second, cannulated reamers in the system allowed repeated corrections to neutral version to be performed over a single orthogonal pin. Third, a single superior and inferior peg configuration for the smaller glenoids simplified the process of sequential reamings to the best fit glenoid. The inferior angled peg and straight superior peg design takes advantage of the greatest bone density in the superior and inferior aspects of the glenoid.

An anatomic study of 412 scapula specimens compared the quality of fit for 6 currently available glenoid designs and found the Solar glenoid to have significantly less mismatch than any other system. The pear-shaped Solar glenoid more closely matched the cortical rim of the native glenoid.25

Although no larger glenoids were required in this study, we planned to eliminate the peripheral pegs in the larger glenoids to standardize all glenoids. The appropriate size drill guide was centered on the glenoid and first the superior and then the inferior peg holes were drilled in a manner similar to clinical implantation. The best fit pegged glenoid trial was then recorded.

A level was applied to the glenoid surface to ensure that the articular surface was held level in the vise. Using a standard drill press a 3.2 mm Steinmann pin was then placed perfectly orthogonal to the articular surface at the exact center point that had been marked.

A right–angled protractor/level was custom-fitted with a 5° wedge along its inferior surface. The slot on the vertical edge of the protractor/level was then held collinear with the centering pin. A 25.4 × 0.9 mm Hall oscillator blade was then held perfectly flush to the 5° wedge attached to the inferior surface of the protractor/level. This allowed an accurate and reproducible method to remove a 5° wedge of bone from the posterior one-half of the glenoid, thus simulating the posterior erosion that occurs typically in osteoarthritis.

The appropriate Solar glenoid reamer was then placed over the centering pin and used to ream the anterior cortex and correct the deformity back to neutral version. The centering pin was removed. A level was used to confirm neutral version. The AP and superoinferior measurements were then repeated. The superior and inferior peg holes were re-drilled using the guide. The glenoid surface trial and pegged trial which fit best was recorded.

This process was repeated to simulate a 10°, 15°, 20°, and 25° deformity until the smallest surface trial had inadequate underlying glenoid bone stock. The degree of deformity correction where peg penetration occurred through the glenoid cortex was recorded. Figure 1 illustrates the sequence of steps.

Figure 1: Sequence of steps used to simulate eccentric posterior glenoid wear

Figure 1: Sequence of steps used to simulate eccentric posterior glenoid wear and the degree of deformity that could be corrected to allow glenoid component placement.

Statistical Method

The change in mean diameter from the baseline AP and superoinferior diameters to the diameters that resulted from reaming to correct a 5°, 10°, 15°, and 20° deformity were compared using the Repeated Measures t test. Using Pearson’s correlation, the angle of deformity that could be corrected was compared to the baseline AP dimension, baseline superoinferior dimension, and the product of the AP and superoinferior dimensions (an area measurement) to determine if the size of the glenoid was related to the ultimate degree of deformity that could be corrected. The baseline AP and superoinferior measurements were then divided at their median. The same was done for the deformity angle distribution. These were placed into a 2×2 table and analyzed by Fisher’s Exact Test to evaluate for trends in glenoid dimension vs ability to correct deformity. The necessity to downsize the glenoid component, inability to place a glenoid component, and peg penetration were collated numerically.

Results

In 4 of 8 specimens, placing a glenoid prosthesis was not possible after correcting a 15° of deformity because of inadequate bone support (N=2), peg penetration (N=1) or both (N=1) (Figure 2). A 20° deformity was correctable in only 2 of 8 specimens and only after downsizing the glenoid component size. With the numbers available, there were no statistically significant differences in superoinferior diameter after glenoid reaming to correct any degree of deformity. The change in AP diameter for each retroversion angle is demonstrated in Figure 3. There was a significant decrease in AP diameter after correction of 10° (P=.006), 15° (P=.0001), and 20° of retroversion (P=.0001).

Figure 2: AP and superoinferior dimensions(mean±SD) after corrective reaming

Figure 2: AP and superoinferior dimensions(mean±SD) after corrective reaming.There is a significant decrease in AP diameter after correction of 10°, 15°, and 20° of retroversion (P=.00635).


Figure 3: Glenoid component size and peg penetration vs. angle of correction

Figure 3: Glenoid component size and peg penetration vs. angle of correction. In 4 of 8 specimens, placing a glenoid prosthesis was not possible after correcting a 15 degree deformity because of inadequate bone support (N=2), peg penetration (N=1), or both (N=1). A 20 degree deformity was correctable in only 2 of 8 specimens and only after downsizing the glenoid component size.

Pearson’s correlation was used to determine if the dimensions of the intact glenoid were related to the ultimate degree of deformity which could be corrected. The angle of deformity that could be corrected was compared to the baseline AP dimension, baseline superoinferior dimension, and the product of the AP and superoinferior dimensions (an area measurement). With the numbers available, none of these could be correlated alone or in combination with the degree of deformity which could be corrected. However, using Fisher’s Exact test, a trend was noticed between baseline AP measurements and the deformity angle distribution. There were 4 cadavers with relatively large diameters at baseline (meaning they were at or above the median of all 8 cadaver measurements). Three of these cadavers had correctable angles of deformity that were at or above the median angle correctable for the 8 cadavers. Of the 4 cadavers with relatively smaller AP diameters, 3 had smaller maximum angles of deformity that could be corrected. For 6 of 8 specimens, “bigger” glenoids allowed a larger angle of deformity to be corrected than “smaller” glenoids. These observations could not achieve statistical significance at P=.05 unless another 16 cadavers were tested and demonstrated the same pattern.

Discussion

Glenoid loosening continues to be a cause of failure in total shoulder arthroplasty.26,27 Correction of glenoid deformity and proper glenoid component placement are considered crucial aspects to prevention of glenoid loosening. Glenoid retroversion, secondary to either eccentric posterior glenoid wear or congenital dysplasia is present in 41% of shoulders undergoing total shoulder arthroplasty.1,28,29 Several authors have attributed posterior instability after total shoulder arthroplasty to excessive, uncorrected glenoid retroversion.17,30-32 An unsupported posterior glenoid rim can lead to micromotion at the prosthesis-host bone junction and can hasten component loosening.5,23 Other work has demonstrated significantly altered glenohumeral kinematics after shoulder arthroplasty performed with a retroverted glenoid component.9,33 In this study, we sought to define the amount of glenoid retroversion that would be amenable to eccentric reaming in a cadaveric model.

There are many proposed solutions to correct excessive glenoid retroversion in total shoulder arthroplasty: augmentation with bone grafting or the glenoid component, placement of the humeral component in less retroversion and eccentric reaming of the glenoid.11,12,20,21 Indications for glenoid bone grafting are considered to be insufficient bone stock or peripheral wear severe enough to result in component malpositioning that could not be corrected by glenoid reaming.3,9,34

Despite the prevalence of posterior glenoid erosion in patients undergoing total shoulder arthroplasty, the percentage of patients requiring correction with internally fixed bone graft has ranged from only 4% (20 of 463) to 10% (9 of 89).11 Although a seemingly sound solution to glenoid deficiency, posterior bone-grafting is a technically demanding procedure. Patients who require glenoid bone-grafting at the time of primary total shoulder arthroplasty may have a tenfold higher rate of glenoid component failure than those who have adequate glenoid version and volume,30 although other investigators have not corroborated this finding.34

Because of the technical complexity and high rate of failure of glenoids that received bone graft, the most common recommended technique to correct eccentric posterior glenoid wear during total shoulder arthroplasty is anterior ”high side” glenoid reaming, yet indications and limitations for this technique are lacking.24 In our study, placing a glenoid prosthesis was not possible in 4 of 8 specimens after correcting a 15° deformity because of inadequate osseous support (N=2), peg penetration (N=1) or both (N=1). Our data suggest therefore that a 15° deformity has only a 50% chance of being correctable by preferential anterior glenoid reaming. A 20° deformity was correctable in only 2 of 8 specimens and only after downsizing the glenoid component size.These findings support those of Clavert et al21 who found that glenoid retroversion greater than 15° cannot be satisfactorily corrected by eccentric reaming.

Our results indicate that correction of as little as a 10° posterior deficiency led to a statistically significant decrease in the AP dimension of the glenoid. A decrease in the AP glenoid width can result in several deleterious consequences. As we have demonstrated in this study, decreased glenoid width can lead to an obligate downsizing of the glenoid component. In addition, as the size of the defect to be corrected increases, the glenoid width is decreased to the point where even the smallest glenoid cannot be placed without overhang and medialization of the glenohumeral joint line. As demonstrated, this medialization can result in peg penetration through the medial scapular neck. Finally, narrowing of the glenoid width may be associated with decreasing stability of the prosthesis.10 In our study, significant AP glenoid narrowing occurred after correcting only a 10° deformity. Thus a 10° deformity may be a more realistic limit of preferential anterior glenoid reaming, beyond which consideration should be given to bone-grafting.

One limitation of this cadaver study was the fact that “neutral version” was assumed for all specimens by placing them level in the vise. A study of 63 normal shoulders by computerized tomography showed that glenoid orientation was a mean of 2° (±5°) of anteversion (range 14° of anteversion to 12° of retroversion) while others have shown a range of “normal” version from 2° of anteversion to 9° of retroversion.3,7,15 Thus, we believe the neutral version used in this study is within the range of accepted normal version. Another limitation was the number of cadavers studied, as additional cadaveric specimen could have increased the power of the study and led to more significant findings. Only one arthroplasty system was tested in this study. Theoretically, other systems may have smaller glenoids available which might fit some of the glenoids which could not be fit with the system used here. Also, the glenoids used in this study were slightly smaller than average.

In a study of 140 shoulders, Iannotti et al35 showed that the average glenoid width was 29 ± 3.2 mm (range, 21-35 mm) and height was 39 ± 3.5 mm (range, 30-48 mm).35 Our eight cadavers had an average width of 26.4 mm (range, 22-30 mm) and average height of 38.5 mm (range, 35-43.9 mm). The defect that we created in this study was based on the glenoid center point in order to standardize the method of defect creation. Therefore, the entire posterior half of the glenoid was involved in the defect. Although this may be an accurate reproduction of larger degree defects, smaller defects probably involve a smaller portion of the glenoid width. As a result of using specimen with no evidence of arthrosis, no other secondary changes (osteophytes, cystic changes) seen in vivo were examined in the present study.

Conclusion

This cadaveric study demonstrated that correction of as little as 10° of posterior glenoid wear by preferential anterior glenoid reaming results in significant narrowing of the glenoid AP distance. Corrective glenoid reaming for wear of >10° results in peg penetration in most glenoids and downsizing of glenoid size for most glenoids. Correction of a 15° deformity results in inability to place a glenoid component in 50% and correction of a 20° deformity is possible in <25% of glenoids.

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Authors

Dr Gillespie is from the Department of Orthopedic Surgery, University Hospitals Case Medical Center, Case Western Reserve University, Cleveland, Ohio; Dr Lyons is from Penn State Milton S. Hershey Medical Center, Hershey, and Dr Lazarus is from the Rothman Institute, Thomas Jefferson University Hospital, Philadelphia, Pennsylvania.

Drs Gillespie and Lyons have no relevant financial relationships to disclose, and Dr Lazarus is a paid consultant for Tornier Inc (Edina, Minnesota).

Correspondence should be addressed to: Robert Gillespie, MD, Department of Orthopedic Surgery, University Hospitals Case Medical Center, Case Western Reserve University, 11100 Euclid Ave, Cleveland, OH 44118.

10.3928/01477447-20090101-07

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