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A radiographic analysis of the effects of prosthesis design on scapular notching following reverse total shoulder arthroplasty
J Shoulder Elbow Surg (2011) 20, 571-576
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A radiographic analysis of the effects of prosthesis design on scapular notching following reverse total shoulder arthroplasty Laurence B. Kempton, MD, Mamtha Balasubramaniam, MS, Elizabeth Ankerson, BS, J. Michael Wiater, MD* William Beaumont Hospital Department of Orthopaedic Surgery, Royal Oak, MI, USA Background: Scapular notching is a well-recognized complication of reverse total shoulder arthroplasty (RTSA). This paper analyzes the effects of prosthesis design on scapular notching in RTSA. Methods: From a database of all RTSA performed by the senior author, shoulders with minimum 12-month follow-up were included (65 shoulders). Notching in shoulders with a prosthesis neck-shaft angle of 155 and no center-of-rotation (COR) offset (group 1) was compared to notching in shoulders with a neck-shaft angle of 143 and 2.5-mm COR offset (group 2). P values less than .05 were considered significant. Results: The notching grade was significantly higher in group 1 than in group 2, even after statistically adjusting for differences in length of follow-up (P ¼ .0081). The incidence of notching in group 1 at 60.7% was significantly higher than the 16.2% incidence in group 2 (P ¼ .0107). Conclusion: Using prostheses with a smaller neck-shaft angle and increased COR offset can significantly reduce scapular notching in RTSA at 1 year, possibly improving prosthesis survival. Level of evidence: Level III, Retrospective Case Control Study, Treatment Study. Ó 2011 Journal of Shoulder and Elbow Surgery Board of Trustees. Keywords: Reverse shoulder arthroplasty; scapular notching; prosthesis design; neck-shaft angle
Since reverse total shoulder arthroplasty (RTSA) was USFDA approved in November 2003, it has rapidly gained in popularity for treatment of patients with cuff tear arthropathy, degenerative arthritis with rotator cuff deficiency, and anterosuperior escape. As experience with RTSA has grown, types and rates of complications from this procedure are being defined.1-3,10,12,15 Scapular notching is a well-recognized complication of RTSA in This study was approved by the William Beaumont Hospital Human Investigation Committee (approval #2006-088). *Reprint requests: J. Michael Wiater, MD, Beverly Hills Orthopaedic Surgery, 17877 West 14 Mile Road, Beverly Hills, MI 48025. E-mail address: [email protected] (J.M. Wiater).
which there is progressive loss of scapular bone starting inferiorly and occasionally continuing superiorly past the glenosphere base plate fixation screws and central post. Over time, this may be associated with base plate loosening, increasing pain, and eventual prosthesis failure requiring either revision or a salvage procedure.11 The initiation of scapular notching is thought to result from mechanical impingement of the superomedial humeral prosthesis on the inferior scapular neck when the arm is adducted.4,6-8,11 Bone loss has often been noted to progress in a time-dependent fashion (presumably due to gradual erosion), and is typically first noted within the first several months postoperatively but has been reported as far postoperatively as 14 months.11 Both notching presence
1058-2746/$ - see front matter Ó 2011 Journal of Shoulder and Elbow Surgery Board of Trustees. doi:10.1016/j.jse.2010.08.024
572 Table I
L.B. Kempton et al. Comparison of patient demographics
Age at time of surgery (average years SD) Male Female Primary arthroplasty Revision arthroplasty Small glenosphere Large glenosphere Mean follow-up (months)
Group 1 155 neck-shaft angle, no COR offset (N ¼ 28)
Group 2 143 neck-shaft angle, 2.5-mm COR offset (N ¼ 37)
P value
71.8 7.2 9 (32.1%) 19 (67.9%) 22 (78.6%) 6 (21.4%) 20 (71.4%) 8 (28.6%) 24
70.7 8.2 15 (40.5%) 22 (59.5%) 32 (86.5%) 5 (13.5%) 28 (75.7%) 9 (24.3%) 16
.57) .61y .51y .78y .0047)
SD, standard deviation. P < .05 considered statistically significant. ) Student’s t test. y 2-tailed Fisher exact test.
and grade have been shown to directly correlate with length of follow-up.7 Further progression beyond the point of mechanical impingement possibly occurs secondary to polyethylene wear-related osteolysis as wear particles are introduced into the notch.7,9 Although scapular notching has not been proven to have a causal relationship with osteolysis, joint capsule inflammation, and component loosening, there is an increased incidence of these phenomena associated with notching, suggesting that they may be direct effects.9 Therefore, to improve RTSA outcomes, there has been a recent movement towards finding ways to avoid the initiation of notching by reducing or eliminating inferior scapular neck impingement. Gutierrez et al in a saw-bones biomechanical study found that decreasing the humeral prosthesis neck-shaft angle, lateralizing the center-of-rotation (COR), and placing the glenosphere in an inferior position on the glenoid allows for increased adduction of the humerus before impingement on the inferior scapular neck occurs.6 Gutierrez et al also showed in a computerized model that center of rotation offset increase, inferior glenosphere placement, inferior glenosphere tilt, humeral neck-shaft angle reduction, and increasing glenosphere diameter all resulted in greater impingement-free range-of-motion.4 The goal of the present investigation was to clinically correlate these laboratory findings with a population of patients undergoing elective RTSA procedures. Our hypothesis was that using a prosthesis with a reduced neck-shaft angle and increased center-of-rotation (COR) offset would decrease the grade and/or incidence of scapular notching.
Materials and methods
shoulders were enrolled. In order to allow time for scapular notching development, minimum follow-up time for inclusion in the study was 12 months, which left 65 shoulders in 64 patients. The mean age was 71.2 years (range, 50-88). The mean follow-up time for all patients was 20 months (range, 12-35). All patients had painful shoulders with rotator cuff pathology. Diagnoses included 29 shoulders with rotator cuff-tear arthropathy, 15 with irreparable rotator cuff tears, 11 with failed humeral head resurfacing (HHR) arthroplasties, 6 with osteoarthritis and concomitant rotator cuff tear, 3 with rheumatoid arthritis, and 1 with a proximal humerus fracture malunion. The 11 shoulders with failed HHR arthroplasties were the only revision cases. See Table I for detailed demographics of patients in each group.
Prosthesis selection In May 2006, the senior author began a series of RTSA using the Tornier Aequalis prosthesis (Tournier Inc., Grenoble, France) with a 155 neck-shaft angle and no COR offset. These shoulders constituted group 1 for data analysis. In order to reduce his use of cemented stems, by late 2006 the senior author began to utilize the Zimmer trabecular metal reverse prosthesis (Zimmer Inc, Warsaw, IN), which has a 143 neck-shaft angle and 2.5-mm COR offset. This system also uses an elevated polyethylene liner of either 7 or 12 . All the patients in this series received the 7 liner, resulting in a cumulative neck-shaft angle of 150 . These shoulders constituted group 2 for data analysis (Figure). The Aequalis prosthesis has 36- and 42-mm glenosphere size options, and the trabecular metal prosthesis has 36- and 40-mm-size options. Based on the patients’ native osteologic sizes, the presence or absence of severe soft tissue and/or bone deficiency, and the senior author’s intraoperative judgment, 48 patients received 36-mm glenospheres, 9 patients received 40-mm glenospheres, and 6 patients received 42-mm glenospheres. We used descriptors in our analysis of ‘‘small’’ for 36 mm and ‘‘large’’ for 40 and 42 mm. See Table I for a stratification of glenosphere size by group.
Patient selection Surgical technique From May 2006 to July 2008, all patients undergoing RTSA by the senior author were offered the chance to enroll in a RTSA outcomes database (137 shoulders). One-hundred thirty-five
The senior author performed all procedures. An attending anesthesiologist administered an interscalene regional block
Prosthesis design affecting scapular notching
Figure Examples of different prosthesis designs. (A) Postoperative AP radiograph of a 155 neck-shaft angle, no COR offset prosthesis. This radiograph also demonstrates grade 2 scapular notching (up to but not past the inferior base plate screw). (B) Postoperative AP radiograph of a 143 , 2.5-mm COR offset prosthesis. preoperatively, and supplementary intravenous sedation or general anesthesia was used intraoperatively according to the circumstances of each case. Beach-chair positioning was utilized. A deltopectoral approach was used with a superior-to-inferior tenotomy of the subscapularis tendon when present, 1 cm medial to the bicipital groove. In most cases, the superior one-third of the pectoralis major tendon was released to aid exposure. The humeral head was osteotomized with approximately 10 of retroversion relative to the forearm axis with the elbow in 90 of flexion. The glenohumeral capsule was released around the entire glenoid in all cases; and the inferior scapular neck origin of the triceps was released, when necessary in tight shoulders, to aid the glenoid exposure. Due to prior studies suggesting that inferior tilt of the glenosphere may improve implant stability and reduce inferior scapular neck impingement,4-6 the glenoid was reamed eccentrically, removing more bone inferiorly so as to create a 10-15 inferior tilt of the base plate. The base plate was also positioned as inferior as possible to allow slight inferior overhang of the glenosphere relative to the scapula, and yet have the base plate fully supported by bone. The base plate was secured with 2 locking and 2 nonlocking screws for the Aequalis prosthesis and 2 locking screws for the trabecular metal prosthesis. The humeral prosthesis and polyethylene liner were placed after the definitive glenosphere was implanted. During closure, the subscapularis tendon was repaired to the tendon stump with nonabsorbable sutures. Soft-tissue-tenodesis of the long head of the biceps tendon when present was performed by incorporating it into the pectoralis major tendon repair. Deep and superficial suction drains were used in all cases.
Postoperative protocol Postoperatively, patients remained in a shoulder immobilizer in slight abduction for 4 weeks. Physical therapy for hand, wrist, and elbow range-of-motion exercises was started on postoperative day number 1, and progressive shoulder range-of-motion was started after 2 weeks.
Radiographic evaluation Patients were asked to follow-up at 2 weeks, 3 months, 6 months, 12 months, and yearly thereafter. At each visit, AP, lateral, and
573 axillary radiographs of the operative shoulder were obtained. All AP radiographs were standardized by placing the x-ray beam tangential to the medial surface of the base plate. Patients with suboptimal AP radiographs underwent repeat x-ray until satisfactory views were obtained. Each patient’s most recent radiographs were reviewed by a fellowship-trained shoulder surgeon to determine the degree of scapular notching based on the Sirveaux grading system7: grade 0: no notch; grade 1: small notch; grade 2: erosion up to inferior screw; grade 3: erosion beyond inferior screw; grade 4: erosion to the central post with lucency under the base plate. The reviewer was blinded to the timing of the surgery but unblinded with regard to prosthetic design, as it was not possible to obscure the prosthetic design on the radiographs and allow enough detail to make the necessary observations. The presence or absence of an inferior scapular neck osteophyte/spur was also noted.
Data analysis Group 1 was compared to group 2 with respect to notching presence, notching grade, and inferior scapular neck spur presence. In addition, the presence of a scapular neck spur was analyzed as a risk factor for presence of notching and grade of notching independent of group assignment. A smaller prosthesis neck-shaft angle has been theorized to be a risk factor for superior dislocation of the prosthesis by allowing the superior pull of the deltoid to more easily disengage the humerosocket from the glenosphere compared to a more horizontal prosthesis neck-shaft angle. To investigate this, we compared dislocation rates in group 1 versus group 2. Demographic data were compared between groups to check for detectable significant differences so that confounders for a multivariate analysis could be identified (Table I).
Statistical methods The 65 shoulders included in this study involved 1 patient with bilateral RTSA. As there are no systemic conditions known to cause bilateral scapular notching, data from this patient’s shoulders were considered to be statistically independent. All variables were first assessed using numerical and graphical techniques to determine if they met the distributional assumptions of the statistical tests being used to analyze them. Based on this preliminary assessment, parametric, nonparametric, or exact statistical tests were used to analyze the data. Continuous variables were summarized using mean standard deviation, minimum, median, and maximum, and were compared using the Wilcoxon two-sample test with the t approximation. Categorical variables were summarized using frequencies and percentages and compared using the 2-tailed Fisher exact test if nominal, or exact Cochran-Armitage trend test if ordinal. In order to account for the influence resulting from the difference in follow-up length between groups, binary responses such as presence/ absence of notching were analyzed using multiple logistic regression, and ordinal multinomial responses such as scapular notching grade were analyzed using cumulative logit models with the logarithmic link based on the multinomial distribution and Likelihood Ratio statistics. P values less than an alpha of .05 (probability of type I error) were considered statistically significant.
574 Table II
L.B. Kempton et al. Prosthesis design as a risk factor for scapular notching Group 1 155 degree neck-shaft angle, no COR offset (N ¼ 28)
Notching present
17 (60.7%)
No notching Grade 1 notching Grade 2 notching Grade 3 notching Grade 4 notching Inferior scapular neck spur present
11 7 9 1 0 8
(39.3%) (25.0%) (32.1%) (3.6%) (28.6%)
Group 2 143 degree neck-shaft angle, 2.5-mm COR offset (N ¼ 37) 6 (16.2%) 31 5 1 0 0 6
(83.8%) (13.5%) (2.7%)
(16.2%)
P value
.0003) .011y <.0001z .0081x
.36)
P < .05 considered statistically significant. ) 2-tailed Fisher exact test. y Adjusted for length of follow-up using type 3 analysis of effects. z Exact Cochran-Armitage trend test. x Adjusted for length of follow-up using logistic regression statistics based on Likelihood Ratio statistics.
Results The incidence of scapular notching in group 1 was 60.7%. This was significantly higher than the 16.2% incidence of scapular notching in group 2 (P ¼ .0003). The significance remained after adjusting for length of follow-up (P ¼ .0107). Patients in group 1 had a significantly higher grade of notching than those in group 2 (P < .0001). Patients in group 1 were 7.7 times more likely to demonstrate notching (independent of grade) than those in group 2. These significant differences remained after statistically adjusting for follow-up length (P ¼ .0081). See Table II for a summary of the analysis. The overall incidence of inferior scapular neck spur formation was 21.5%. There was no difference detected between groups 1 and 2 when looking at the presence versus absence of an inferior scapular neck osteophyte (Table II). The presence of an inferior scapular neck osteophyte was predictive of neither notching grade (P ¼ .2454, Cochran-Armitage trend test) nor notching presence independent of grade (P ¼ .1640, logistic regression statistics for type 3 analysis). No patients in group 1 and 1 patient in group 2 had episodes of prosthetic instability.
Prosthetic survivorship No prostheses in either group have been revised because of prosthetic loosening.
Discussion Biomechanical and computerized models have previously shown that inferiorly tilting the glenosphere, reducing the humeral neck-shaft angle, increasing the COR offset,
increasing the glenosphere diameter, and inferiorly translating the glenosphere increase impingement-free adduction range-of-motion in RTSA.4-6 Therefore, we logically hypothesized that decreasing the humeral neck-shaft angle and increasing the COR offset would decrease scapular notching. Prior clinical studies with minimum follow-up times ranging from 3 months to 2 years have shown scapular notching rates ranging from 0% to 96%.3,7,11,12,14,15 In our series of patients with minimum 12-month follow-up, we found a significant reduction in notching incidence from 61% to 16% and in the grade of notching with a decreased neckshaft angle and increased COR offset. This significance remained after statistically adjusting for length of follow-up. Prior studies have noted the presence of inferior scapular neck spurs or osteophytes and found correlations between their presence and scapular notching;11,13 however, we found no such correlation in this series. Although these formations have been hypothesized to result from traction of the long head of the triceps,11 their etiology is uncertain. Another possible explanation is a reactive ossification or remodeling reaction secondary to adduction impingement. To our knowledge, such osteophytes have not been found to have any clinical consequences, and we plan to look for possible correlations in future outcomes studies. Despite a theoretical increased risk for superior instability of the prosthesis with a smaller neck-shaft angle, we did not observe this outcome. However, even if a statistically significant difference would be found with a larger study population, our dislocation/subluxation rate was acceptably low; therefore, the number of patients that would be required to detect this difference would likely make it a clinically irrelevant issue. Because both the neck-shaft angle and COR offset were different between groups 1 and 2, it is unclear which of these factors had a greater influence on scapular notching. In their simulation study, Gutierrez et al concluded that neck-shaft
Prosthesis design affecting scapular notching angle affects ‘‘adduction deficit’’ more substantially than COR offset.4 However, they were testing COR offsets ranging from 0-10 mm and neck-shaft angles ranging from 170 to 130 , which are much larger ranges than our respective 0-2.5 mm and 155-143 . They also found that there were cumulative effects when combining different neck-shaft angles, superior/inferior glenosphere positions, amounts of glenosphere tilt, values of COR offset, and glenosphere sizes; therefore, we assume that the combination of increased COR offset and decreased neck-shaft angle had cumulative (not necessarily additive) effects in our series. The most significant limitation of this study is the lack of randomization and consequent difference in length of follow-up between groups. This discrepancy would likely be more pronounced when comparing grades of notching between groups versus comparing incidence (grade-independent) of notching, because once notching is initiated it may progress to higher grades over time. Without consideration of grade, the number of shoulders with notching presence would be much less affected by a temporal difference, because we excluded patients with less than 12 months follow-up, thus giving all patients time to initiate notching. As mentioned above, we also statistically adjusted for differences in follow-up length. Lack of randomization may also have resulted in patients in the later group being treated with an improved technique due to surgeon experience, which potentially contributed to a reduction in scapular notching. However, other than placement of the base plate low on the face of the glenoid,11 we are unaware of any improvement in surgical technique that has been shown to decrease the likelihood of scapular notching. The 12-month minimum follow-up may be considered a limitation of this study, because it is too early to analyze mid- to long-term clinical results. However, looking at functional and subjective outcomes was outside the scope of this study. Scapular notching usually starts long before 12 months postoperatively; therefore, our study design gave patients ample time to initiate notching. Although the shoulders with notching will likely demonstrate notching progression with time, very few (if any) shoulders that have avoided notching by 12 months are expected to begin notching later.11 The effect of the 7 elevated polyethylene liner in the trabecular metal reverse prosthesis is not known. It brings the final neck-shaft angle up to 150 , which is still less than the 155 in the Aequalis prosthesis. It is unclear whether the 143 neck-shaft angle of the metal or the 150 neck-shaft angle of the metal and polyethylene combination is what affects adduction impingement. It is beyond debate that impingement of a metal prosthesis on bone could lead to notching of the bone, as the metal is harder and more durable than cortical bone. However, it is debatable whether impingement of polyethylene, which has a lower modulus of elasticity and less hardness, on cortical bone could lead to notching of the bone. In addition, the senior author and
575 others have observed retrieved RTSA polyethylene liners at revision that have demonstrated significant notching and wear of the medial polyethylene.9 Thus it is possible that the mechanical impingement first leads to notching of the polyethylene by the bone. Then, once the metal prosthesis begins to impinge, it leads to notching of the bone by the metal. For these reasons, we have chosen to view the 143 neck-shaft angle of the metal prosthesis as the effective neck-shaft angle. Polyethylene wear-related osteolysis might play a role in scapular notching, as the polyethylene erodes upon contact with the lateral scapula. If the liner wears rapidly enough, then humeral prosthesis impingement, rather than the polyethylene, would limit adduction and initiate notching. The polyethylene may then exacerbate notching via osteolysis.
Conclusion In summary, we found a significant reduction in incidence and grade of scapular notching with a reduction in prosthesis neck-shaft angle and an increase in COR offset, thus clinically confirming the predictions of prior biomechanical and anatomic modeling studies. We did not correlate radiographic outcomes with clinical outcomes such as range of motion, patient satisfaction, prosthesis loosening, or prosthesis survivorship; although these data have been collected as part of a prospective outcomes study. There have been no patients in this series with evidence of prosthesis loosening. Future study in this area should determine whether scapular notching results in higher rates of loosening, pain, and/or reoperation, and whether alterations in prosthesis design significantly affect patient satisfaction and range of motion.
Disclaimer J. Michael Wiater, MD is a paid consultant for Zimmer, Inc. The rest of the authors, their immediate families, and any research foundations with which they are affiliated have not received any financial payments or other benefits from any commercial entity related to the subject of this article. No outside funding or grants were received that assisted this study.
References 1. Boileau P, Watkinson DJ, Hatzidakis AM, Balg F. Grammont reverse prosthesis: design, rationale, and biomechanics. J Shoulder Elbow Surg 2005;14(1 SupplS):147S-61. doi:10.1016/j.jse.2004.10.006 2. Boulahia A, Edwards TB, Walch G, Baratta RV. Early results of a reverse design prosthesis in the treatment of arthritis of the shoulder
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in elderly patients with a large rotator cuff tear. Orthopedics 2002;25: 129-33. PMid:11866145. Frankle M, Siegal S, Pupello D, Saleem A, Mighell M, Vasey M. The reverse shoulder prosthesis for glenohumeral arthritis associated with severe rotator cuff deficiency. A minimum two-year follow-up study of sixty patients. J Bone Joint Surg Am 2005;87:1697-705. doi:10.2106/ JBJS.D.02813 Gutierrez S, Comiskey CA IV, Luo ZP, Pupello DR, Frankle MA. Range of impingement-free abduction and adduction deficit after reverse shoulder arthroplasty. Hierarchy of surgical and implantdesign-related factors. J Bone Joint Surg Am 2008;90:2606-15. doi: 10.2106/JBJS.H.00012 Gutierrez S, Greiwe RM, Frankle MA, Siegal S, Lee WE III. Biomechanical comparison of component position and hardware failure in the reverse shoulder prosthesis. J Shoulder Elbow Surg 2007; 16(3 Suppl):S9-12. doi:10.1016/j.jse.2005.11.008 Gutierrez S, Levy JC, Frankle MA, Cuff D, Keller TS, Pupello DR, et al. Evaluation of abduction range of motion and avoidance of inferior scapular impingement in a reverse shoulder model. J Shoulder Elbow Surg 2008;17:608-15. doi:10.1016/j.jse.2007.11.010 Levigne C, Boileau P, Favard L, Garaud P, Mole D, Sirveaux F, et al. Scapular notching in reverse shoulder arthroplasty. J Shoulder Elbow Surg 2008;17:925-35. doi:10.1016/j.jse.2008.02.010 Nyffeler RW, Werner CM, Gerber C. Biomechanical relevance of glenoid component positioning in the reverse Delta III total shoulder prosthesis. J Shoulder Elbow Surg 2005;14:524-8. doi:10.1016/j.jse. 2004.09.010
L.B. Kempton et al. 9. Nyffeler RW, Werner CM, Simmen BR, Gerber C. Analysis of a retrieved Delta III total shoulder prosthesis. J Bone Joint Surg Br 2004;86:1187-91. doi:10.1302/0301-620X.86B8.15228 10. Rittmeister M, Kerschbaumer F. Grammont reverse total shoulder arthroplasty in patients with rheumatoid arthritis and nonreconstructible rotator cuff lesions. J Shoulder Elbow Surg 2001;10: 17-22. doi:10.1067/mse.2001.110515 11. Simovitch RW, Zumstein MA, Lohri E, Helmy N, Gerber C. Predictors of scapular notching in patients managed with the Delta III reverse total shoulder replacement. J Bone Joint Surg Am 2007;89:588-600. doi:10.2106/JBJS.F.00226 12. Sirveaux F, Favard L, Oudet D, Huquet D, Walch G, Mole D. Grammont inverted total shoulder arthroplasty in the treatment of glenohumeral osteoarthritis with massive rupture of the cuff. Results of a multicentre study of 80 shoulders J Bone Joint Surg Br 2004;86: 388-95. doi:10.1302/0301-620X.86B3.14024 13. Valenti PH, Boutens D, Nerot C. Delta 3 reverse prosthesis for osteoarthritis with massive rotator cuff tear: long term results (>5 years). In: Walch G, Boileau P, Mole D, editors. Shoulder prosthesis: Two to ten year follow-up. Montpellier, France: Sauramps Medical; 2001. p. 253-9. 14. Wall B, Nove-Josserand L, O’Connor DP, Edwards TB, Walch G. Reverse total shoulder arthroplasty: A review of results according to etiology. J Bone Joint Surg Am 2007;89:1476-85. doi:10.2106/JBJS.F.00666 15. Werner CM, Steinmann PA, Gilbart M, Gerber C. Treatment of painful pseudoparesis due to irreparable rotator cuff dysfunction with the Delta III reverse-ball-andsocket total shoulder prosthesis. J Bone Joint Surg Am 2005;87:1476-86. doi:10.2106/JBJS.D.02342
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A radiographic analysis of the effects of prosthesis design on scapular notching following reverse total shoulder arthroplasty Laurence B. Kempton, MD, Mamtha Balasubramaniam, MS, Elizabeth Ankerson, BS, J. Michael Wiater, MD* William Beaumont Hospital Department of Orthopaedic Surgery, Royal Oak, MI, USA Background: Scapular notching is a well-recognized complication of reverse total shoulder arthroplasty (RTSA). This paper analyzes the effects of prosthesis design on scapular notching in RTSA. Methods: From a database of all RTSA performed by the senior author, shoulders with minimum 12-month follow-up were included (65 shoulders). Notching in shoulders with a prosthesis neck-shaft angle of 155 and no center-of-rotation (COR) offset (group 1) was compared to notching in shoulders with a neck-shaft angle of 143 and 2.5-mm COR offset (group 2). P values less than .05 were considered significant. Results: The notching grade was significantly higher in group 1 than in group 2, even after statistically adjusting for differences in length of follow-up (P ¼ .0081). The incidence of notching in group 1 at 60.7% was significantly higher than the 16.2% incidence in group 2 (P ¼ .0107). Conclusion: Using prostheses with a smaller neck-shaft angle and increased COR offset can significantly reduce scapular notching in RTSA at 1 year, possibly improving prosthesis survival. Level of evidence: Level III, Retrospective Case Control Study, Treatment Study. Ó 2011 Journal of Shoulder and Elbow Surgery Board of Trustees. Keywords: Reverse shoulder arthroplasty; scapular notching; prosthesis design; neck-shaft angle
Since reverse total shoulder arthroplasty (RTSA) was USFDA approved in November 2003, it has rapidly gained in popularity for treatment of patients with cuff tear arthropathy, degenerative arthritis with rotator cuff deficiency, and anterosuperior escape. As experience with RTSA has grown, types and rates of complications from this procedure are being defined.1-3,10,12,15 Scapular notching is a well-recognized complication of RTSA in This study was approved by the William Beaumont Hospital Human Investigation Committee (approval #2006-088). *Reprint requests: J. Michael Wiater, MD, Beverly Hills Orthopaedic Surgery, 17877 West 14 Mile Road, Beverly Hills, MI 48025. E-mail address: [email protected] (J.M. Wiater).
which there is progressive loss of scapular bone starting inferiorly and occasionally continuing superiorly past the glenosphere base plate fixation screws and central post. Over time, this may be associated with base plate loosening, increasing pain, and eventual prosthesis failure requiring either revision or a salvage procedure.11 The initiation of scapular notching is thought to result from mechanical impingement of the superomedial humeral prosthesis on the inferior scapular neck when the arm is adducted.4,6-8,11 Bone loss has often been noted to progress in a time-dependent fashion (presumably due to gradual erosion), and is typically first noted within the first several months postoperatively but has been reported as far postoperatively as 14 months.11 Both notching presence
1058-2746/$ - see front matter Ó 2011 Journal of Shoulder and Elbow Surgery Board of Trustees. doi:10.1016/j.jse.2010.08.024
572 Table I
L.B. Kempton et al. Comparison of patient demographics
Age at time of surgery (average years SD) Male Female Primary arthroplasty Revision arthroplasty Small glenosphere Large glenosphere Mean follow-up (months)
Group 1 155 neck-shaft angle, no COR offset (N ¼ 28)
Group 2 143 neck-shaft angle, 2.5-mm COR offset (N ¼ 37)
P value
71.8 7.2 9 (32.1%) 19 (67.9%) 22 (78.6%) 6 (21.4%) 20 (71.4%) 8 (28.6%) 24
70.7 8.2 15 (40.5%) 22 (59.5%) 32 (86.5%) 5 (13.5%) 28 (75.7%) 9 (24.3%) 16
.57) .61y .51y .78y .0047)
SD, standard deviation. P < .05 considered statistically significant. ) Student’s t test. y 2-tailed Fisher exact test.
and grade have been shown to directly correlate with length of follow-up.7 Further progression beyond the point of mechanical impingement possibly occurs secondary to polyethylene wear-related osteolysis as wear particles are introduced into the notch.7,9 Although scapular notching has not been proven to have a causal relationship with osteolysis, joint capsule inflammation, and component loosening, there is an increased incidence of these phenomena associated with notching, suggesting that they may be direct effects.9 Therefore, to improve RTSA outcomes, there has been a recent movement towards finding ways to avoid the initiation of notching by reducing or eliminating inferior scapular neck impingement. Gutierrez et al in a saw-bones biomechanical study found that decreasing the humeral prosthesis neck-shaft angle, lateralizing the center-of-rotation (COR), and placing the glenosphere in an inferior position on the glenoid allows for increased adduction of the humerus before impingement on the inferior scapular neck occurs.6 Gutierrez et al also showed in a computerized model that center of rotation offset increase, inferior glenosphere placement, inferior glenosphere tilt, humeral neck-shaft angle reduction, and increasing glenosphere diameter all resulted in greater impingement-free range-of-motion.4 The goal of the present investigation was to clinically correlate these laboratory findings with a population of patients undergoing elective RTSA procedures. Our hypothesis was that using a prosthesis with a reduced neck-shaft angle and increased center-of-rotation (COR) offset would decrease the grade and/or incidence of scapular notching.
Materials and methods
shoulders were enrolled. In order to allow time for scapular notching development, minimum follow-up time for inclusion in the study was 12 months, which left 65 shoulders in 64 patients. The mean age was 71.2 years (range, 50-88). The mean follow-up time for all patients was 20 months (range, 12-35). All patients had painful shoulders with rotator cuff pathology. Diagnoses included 29 shoulders with rotator cuff-tear arthropathy, 15 with irreparable rotator cuff tears, 11 with failed humeral head resurfacing (HHR) arthroplasties, 6 with osteoarthritis and concomitant rotator cuff tear, 3 with rheumatoid arthritis, and 1 with a proximal humerus fracture malunion. The 11 shoulders with failed HHR arthroplasties were the only revision cases. See Table I for detailed demographics of patients in each group.
Prosthesis selection In May 2006, the senior author began a series of RTSA using the Tornier Aequalis prosthesis (Tournier Inc., Grenoble, France) with a 155 neck-shaft angle and no COR offset. These shoulders constituted group 1 for data analysis. In order to reduce his use of cemented stems, by late 2006 the senior author began to utilize the Zimmer trabecular metal reverse prosthesis (Zimmer Inc, Warsaw, IN), which has a 143 neck-shaft angle and 2.5-mm COR offset. This system also uses an elevated polyethylene liner of either 7 or 12 . All the patients in this series received the 7 liner, resulting in a cumulative neck-shaft angle of 150 . These shoulders constituted group 2 for data analysis (Figure). The Aequalis prosthesis has 36- and 42-mm glenosphere size options, and the trabecular metal prosthesis has 36- and 40-mm-size options. Based on the patients’ native osteologic sizes, the presence or absence of severe soft tissue and/or bone deficiency, and the senior author’s intraoperative judgment, 48 patients received 36-mm glenospheres, 9 patients received 40-mm glenospheres, and 6 patients received 42-mm glenospheres. We used descriptors in our analysis of ‘‘small’’ for 36 mm and ‘‘large’’ for 40 and 42 mm. See Table I for a stratification of glenosphere size by group.
Patient selection Surgical technique From May 2006 to July 2008, all patients undergoing RTSA by the senior author were offered the chance to enroll in a RTSA outcomes database (137 shoulders). One-hundred thirty-five
The senior author performed all procedures. An attending anesthesiologist administered an interscalene regional block
Prosthesis design affecting scapular notching
Figure Examples of different prosthesis designs. (A) Postoperative AP radiograph of a 155 neck-shaft angle, no COR offset prosthesis. This radiograph also demonstrates grade 2 scapular notching (up to but not past the inferior base plate screw). (B) Postoperative AP radiograph of a 143 , 2.5-mm COR offset prosthesis. preoperatively, and supplementary intravenous sedation or general anesthesia was used intraoperatively according to the circumstances of each case. Beach-chair positioning was utilized. A deltopectoral approach was used with a superior-to-inferior tenotomy of the subscapularis tendon when present, 1 cm medial to the bicipital groove. In most cases, the superior one-third of the pectoralis major tendon was released to aid exposure. The humeral head was osteotomized with approximately 10 of retroversion relative to the forearm axis with the elbow in 90 of flexion. The glenohumeral capsule was released around the entire glenoid in all cases; and the inferior scapular neck origin of the triceps was released, when necessary in tight shoulders, to aid the glenoid exposure. Due to prior studies suggesting that inferior tilt of the glenosphere may improve implant stability and reduce inferior scapular neck impingement,4-6 the glenoid was reamed eccentrically, removing more bone inferiorly so as to create a 10-15 inferior tilt of the base plate. The base plate was also positioned as inferior as possible to allow slight inferior overhang of the glenosphere relative to the scapula, and yet have the base plate fully supported by bone. The base plate was secured with 2 locking and 2 nonlocking screws for the Aequalis prosthesis and 2 locking screws for the trabecular metal prosthesis. The humeral prosthesis and polyethylene liner were placed after the definitive glenosphere was implanted. During closure, the subscapularis tendon was repaired to the tendon stump with nonabsorbable sutures. Soft-tissue-tenodesis of the long head of the biceps tendon when present was performed by incorporating it into the pectoralis major tendon repair. Deep and superficial suction drains were used in all cases.
Postoperative protocol Postoperatively, patients remained in a shoulder immobilizer in slight abduction for 4 weeks. Physical therapy for hand, wrist, and elbow range-of-motion exercises was started on postoperative day number 1, and progressive shoulder range-of-motion was started after 2 weeks.
Radiographic evaluation Patients were asked to follow-up at 2 weeks, 3 months, 6 months, 12 months, and yearly thereafter. At each visit, AP, lateral, and
573 axillary radiographs of the operative shoulder were obtained. All AP radiographs were standardized by placing the x-ray beam tangential to the medial surface of the base plate. Patients with suboptimal AP radiographs underwent repeat x-ray until satisfactory views were obtained. Each patient’s most recent radiographs were reviewed by a fellowship-trained shoulder surgeon to determine the degree of scapular notching based on the Sirveaux grading system7: grade 0: no notch; grade 1: small notch; grade 2: erosion up to inferior screw; grade 3: erosion beyond inferior screw; grade 4: erosion to the central post with lucency under the base plate. The reviewer was blinded to the timing of the surgery but unblinded with regard to prosthetic design, as it was not possible to obscure the prosthetic design on the radiographs and allow enough detail to make the necessary observations. The presence or absence of an inferior scapular neck osteophyte/spur was also noted.
Data analysis Group 1 was compared to group 2 with respect to notching presence, notching grade, and inferior scapular neck spur presence. In addition, the presence of a scapular neck spur was analyzed as a risk factor for presence of notching and grade of notching independent of group assignment. A smaller prosthesis neck-shaft angle has been theorized to be a risk factor for superior dislocation of the prosthesis by allowing the superior pull of the deltoid to more easily disengage the humerosocket from the glenosphere compared to a more horizontal prosthesis neck-shaft angle. To investigate this, we compared dislocation rates in group 1 versus group 2. Demographic data were compared between groups to check for detectable significant differences so that confounders for a multivariate analysis could be identified (Table I).
Statistical methods The 65 shoulders included in this study involved 1 patient with bilateral RTSA. As there are no systemic conditions known to cause bilateral scapular notching, data from this patient’s shoulders were considered to be statistically independent. All variables were first assessed using numerical and graphical techniques to determine if they met the distributional assumptions of the statistical tests being used to analyze them. Based on this preliminary assessment, parametric, nonparametric, or exact statistical tests were used to analyze the data. Continuous variables were summarized using mean standard deviation, minimum, median, and maximum, and were compared using the Wilcoxon two-sample test with the t approximation. Categorical variables were summarized using frequencies and percentages and compared using the 2-tailed Fisher exact test if nominal, or exact Cochran-Armitage trend test if ordinal. In order to account for the influence resulting from the difference in follow-up length between groups, binary responses such as presence/ absence of notching were analyzed using multiple logistic regression, and ordinal multinomial responses such as scapular notching grade were analyzed using cumulative logit models with the logarithmic link based on the multinomial distribution and Likelihood Ratio statistics. P values less than an alpha of .05 (probability of type I error) were considered statistically significant.
574 Table II
L.B. Kempton et al. Prosthesis design as a risk factor for scapular notching Group 1 155 degree neck-shaft angle, no COR offset (N ¼ 28)
Notching present
17 (60.7%)
No notching Grade 1 notching Grade 2 notching Grade 3 notching Grade 4 notching Inferior scapular neck spur present
11 7 9 1 0 8
(39.3%) (25.0%) (32.1%) (3.6%) (28.6%)
Group 2 143 degree neck-shaft angle, 2.5-mm COR offset (N ¼ 37) 6 (16.2%) 31 5 1 0 0 6
(83.8%) (13.5%) (2.7%)
(16.2%)
P value
.0003) .011y <.0001z .0081x
.36)
P < .05 considered statistically significant. ) 2-tailed Fisher exact test. y Adjusted for length of follow-up using type 3 analysis of effects. z Exact Cochran-Armitage trend test. x Adjusted for length of follow-up using logistic regression statistics based on Likelihood Ratio statistics.
Results The incidence of scapular notching in group 1 was 60.7%. This was significantly higher than the 16.2% incidence of scapular notching in group 2 (P ¼ .0003). The significance remained after adjusting for length of follow-up (P ¼ .0107). Patients in group 1 had a significantly higher grade of notching than those in group 2 (P < .0001). Patients in group 1 were 7.7 times more likely to demonstrate notching (independent of grade) than those in group 2. These significant differences remained after statistically adjusting for follow-up length (P ¼ .0081). See Table II for a summary of the analysis. The overall incidence of inferior scapular neck spur formation was 21.5%. There was no difference detected between groups 1 and 2 when looking at the presence versus absence of an inferior scapular neck osteophyte (Table II). The presence of an inferior scapular neck osteophyte was predictive of neither notching grade (P ¼ .2454, Cochran-Armitage trend test) nor notching presence independent of grade (P ¼ .1640, logistic regression statistics for type 3 analysis). No patients in group 1 and 1 patient in group 2 had episodes of prosthetic instability.
Prosthetic survivorship No prostheses in either group have been revised because of prosthetic loosening.
Discussion Biomechanical and computerized models have previously shown that inferiorly tilting the glenosphere, reducing the humeral neck-shaft angle, increasing the COR offset,
increasing the glenosphere diameter, and inferiorly translating the glenosphere increase impingement-free adduction range-of-motion in RTSA.4-6 Therefore, we logically hypothesized that decreasing the humeral neck-shaft angle and increasing the COR offset would decrease scapular notching. Prior clinical studies with minimum follow-up times ranging from 3 months to 2 years have shown scapular notching rates ranging from 0% to 96%.3,7,11,12,14,15 In our series of patients with minimum 12-month follow-up, we found a significant reduction in notching incidence from 61% to 16% and in the grade of notching with a decreased neckshaft angle and increased COR offset. This significance remained after statistically adjusting for length of follow-up. Prior studies have noted the presence of inferior scapular neck spurs or osteophytes and found correlations between their presence and scapular notching;11,13 however, we found no such correlation in this series. Although these formations have been hypothesized to result from traction of the long head of the triceps,11 their etiology is uncertain. Another possible explanation is a reactive ossification or remodeling reaction secondary to adduction impingement. To our knowledge, such osteophytes have not been found to have any clinical consequences, and we plan to look for possible correlations in future outcomes studies. Despite a theoretical increased risk for superior instability of the prosthesis with a smaller neck-shaft angle, we did not observe this outcome. However, even if a statistically significant difference would be found with a larger study population, our dislocation/subluxation rate was acceptably low; therefore, the number of patients that would be required to detect this difference would likely make it a clinically irrelevant issue. Because both the neck-shaft angle and COR offset were different between groups 1 and 2, it is unclear which of these factors had a greater influence on scapular notching. In their simulation study, Gutierrez et al concluded that neck-shaft
Prosthesis design affecting scapular notching angle affects ‘‘adduction deficit’’ more substantially than COR offset.4 However, they were testing COR offsets ranging from 0-10 mm and neck-shaft angles ranging from 170 to 130 , which are much larger ranges than our respective 0-2.5 mm and 155-143 . They also found that there were cumulative effects when combining different neck-shaft angles, superior/inferior glenosphere positions, amounts of glenosphere tilt, values of COR offset, and glenosphere sizes; therefore, we assume that the combination of increased COR offset and decreased neck-shaft angle had cumulative (not necessarily additive) effects in our series. The most significant limitation of this study is the lack of randomization and consequent difference in length of follow-up between groups. This discrepancy would likely be more pronounced when comparing grades of notching between groups versus comparing incidence (grade-independent) of notching, because once notching is initiated it may progress to higher grades over time. Without consideration of grade, the number of shoulders with notching presence would be much less affected by a temporal difference, because we excluded patients with less than 12 months follow-up, thus giving all patients time to initiate notching. As mentioned above, we also statistically adjusted for differences in follow-up length. Lack of randomization may also have resulted in patients in the later group being treated with an improved technique due to surgeon experience, which potentially contributed to a reduction in scapular notching. However, other than placement of the base plate low on the face of the glenoid,11 we are unaware of any improvement in surgical technique that has been shown to decrease the likelihood of scapular notching. The 12-month minimum follow-up may be considered a limitation of this study, because it is too early to analyze mid- to long-term clinical results. However, looking at functional and subjective outcomes was outside the scope of this study. Scapular notching usually starts long before 12 months postoperatively; therefore, our study design gave patients ample time to initiate notching. Although the shoulders with notching will likely demonstrate notching progression with time, very few (if any) shoulders that have avoided notching by 12 months are expected to begin notching later.11 The effect of the 7 elevated polyethylene liner in the trabecular metal reverse prosthesis is not known. It brings the final neck-shaft angle up to 150 , which is still less than the 155 in the Aequalis prosthesis. It is unclear whether the 143 neck-shaft angle of the metal or the 150 neck-shaft angle of the metal and polyethylene combination is what affects adduction impingement. It is beyond debate that impingement of a metal prosthesis on bone could lead to notching of the bone, as the metal is harder and more durable than cortical bone. However, it is debatable whether impingement of polyethylene, which has a lower modulus of elasticity and less hardness, on cortical bone could lead to notching of the bone. In addition, the senior author and
575 others have observed retrieved RTSA polyethylene liners at revision that have demonstrated significant notching and wear of the medial polyethylene.9 Thus it is possible that the mechanical impingement first leads to notching of the polyethylene by the bone. Then, once the metal prosthesis begins to impinge, it leads to notching of the bone by the metal. For these reasons, we have chosen to view the 143 neck-shaft angle of the metal prosthesis as the effective neck-shaft angle. Polyethylene wear-related osteolysis might play a role in scapular notching, as the polyethylene erodes upon contact with the lateral scapula. If the liner wears rapidly enough, then humeral prosthesis impingement, rather than the polyethylene, would limit adduction and initiate notching. The polyethylene may then exacerbate notching via osteolysis.
Conclusion In summary, we found a significant reduction in incidence and grade of scapular notching with a reduction in prosthesis neck-shaft angle and an increase in COR offset, thus clinically confirming the predictions of prior biomechanical and anatomic modeling studies. We did not correlate radiographic outcomes with clinical outcomes such as range of motion, patient satisfaction, prosthesis loosening, or prosthesis survivorship; although these data have been collected as part of a prospective outcomes study. There have been no patients in this series with evidence of prosthesis loosening. Future study in this area should determine whether scapular notching results in higher rates of loosening, pain, and/or reoperation, and whether alterations in prosthesis design significantly affect patient satisfaction and range of motion.
Disclaimer J. Michael Wiater, MD is a paid consultant for Zimmer, Inc. The rest of the authors, their immediate families, and any research foundations with which they are affiliated have not received any financial payments or other benefits from any commercial entity related to the subject of this article. No outside funding or grants were received that assisted this study.
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