Carol Major

Farzana Ansari

Lisa Pruitt

MD Michael Ries

MD Stephen B Gunther

MD Tom R Norris

Michael Ries

MS Farzana Ansari

PhD Lisa Pruitt

Stephen B Gunther

Tom R Norris

CiteSeerX

Copyright © 2013 by ASME 1 
INTRODUCTION 
 Reverse total shoulder arthroplasty (RSA) serves as an effective 
alternative to total shoulder arthroplasty (TSA), especially for patients 
with rotator cuff deficiency [1]. Long-term performance of RSA has 
been limited by a variety of complications including dislocation, 
infection, humeral fracture, glenoid loosening, glenoid unscrewing, 
scapular erosion and polyethylene debris [1-2].  Dissociation between 
the proximal metaphyseal component and distal humeral prosthetic 
stem (disaphysis) in modular RSAs has been reported with low 
frequency (1-2%) [2]; however, cases have led to severe consequences 
including in vivo disassembly [3]. Most cases of unscrewing have 
occurred due to insufficient support for the prosthesis resulting from 
proximal humeral bone deficiency [4-5]. In this study, we evaluate a 
retrieved RSA that unscrewed and subsequently fractured in vivo in a 
patient with proximal bone deficiency.     
METHODS 
 A modular Tornier Aequalis RSA which relies on screw fixation 
of the proximal body to the distal stem was retrieved (August 2011) 
from the left shoulder of a 60 yr old male patient (Figure 1). At the 
time of implantation (March 2007), the proximal bone was noted as 
deficient above the metaphyseal-diaphysis junction. The patient 
experienced episodes during which his shoulder was caught in external 
rotation and had to be manually rotated back in place to restore 
function. The device failed when the screw junction between the 
metaphysis and diaphysis fractured while the patient was golfing. The 
implant was immediately retrieved. Extensive metallosis was observed 
throughout the soft tissue and down the humeral canal.  The fracture 
occurred just above the remaining humeral bone shaft. Grade II 
scapular notching was noted. There was no bone or soft tissue support 
fixed to the implant above the metaphysis-diaphysis junction.   
 Optical microscopy and scanning electron microscopy (SEM) 
were used to characterize the fracture surfaces of the retrieval. Failure 
analysis utilizing composite beam theory under pure bending was 
performed to evaluate the stresses in the component with and without 
proximal bone support.  An annulus cross section was used to model 
the composite beam, as seen in Figure 2. Stresses were calculated as: 
𝝈𝒔 =
𝑬𝒔𝑴𝒚
∑𝑬𝒊𝑰𝒊
[Eq. 1] 
where σs is the maximum normal stress in the metaphyseal screw and 
Es is its elastic modulus; M is the bending moment applied about the 
neutral axis of the composite beam; y is the maximum distance from 
the neutral axis of the composite beam (the radius of the stem); Ei is 
the elastic modulus of contributing material; and Ii is the second 
moment of inertia for each material cross section (Table 1).  
RESULTS 
 Optical and metric analysis of the retrieval confirmed that 
unscrewing occurred before fracture of the metaphyseal screw. 
Measurements of the remaining screw lodged in the diaphysis and of 
the fractured screw exposed a gap between the superior surface of the 
humeral stem and the inferior mating surface of the metaphyseal 
component just before fracture. Removal of the remaining screw from 
the humeral stem revealed that the fracture had occurred at a 3mm 
diameter through-hole in the metaphyseal screw (Figure 3). Optical 
microscopy of the screw revealed some fretting wear of threads which 
may have contributed to the extensive metallosis observed at retrieval.  
 Composite beam analysis compared stresses in the humeral stem 
(assuming no unscrewing had occurred and ignoring any stress 
concentrations due to the through-hole) with and without proximal 
bone support. Under the same dimensions and bending moment 
DESIGN ANALYSIS: MODULAR REVERSE TOTAL SHOULDER PROSTHETIC FAILURE 
WITHOUT PROXIMAL BONE SUPPORT 
1Farzana Ansari, MS, 1Carol Major, 2Stephen B. Gunther, MD, 3Tom R. Norris, MD, 4Michael 
Ries, MD, and 1Lisa Pruitt, PhD 
1 Department of Mechanical Engineering 
University of California, Berkeley 
Berkeley, California 94709 
USA 
 
2 Department of Orthopedic Surgery 
Martha Jefferson Hospital 
Charlottesville, VA 22911 
USA 
 3 Department of Orthopedic Surgery 
San Francisco Shoulder, Elbow and Hand Clinic 
San Francisco, California 94115 
USA 
4 Department of Orthopedic Surgery 
University of California, San Francisco 
San Francisco, California 94122 
USA 
 
Proceedings of the ASME 2013 Summer Bioengineering Conference 
SBC2013 
June 26-29, 2013, Sunriver, Oregon, USA 
SBC2013-14293
Downloaded From: https://proceedings.asmedigitalcollection.asme.org on 07/01/2019 Terms of Use: http://www.asme.org/about-asme/terms-of-use
Copyright © 2013 by ASME 2 
applied, proximal bone deficiency was shown to increase stresses in 
the implant by 60%. Unscrewing would reduce the cross section 
geometry and thus exponentially decrease the moment of inertia of the 
humeral stem, presumably driving the stresses even higher as shown 
by Eq. 1. Addition of a through-hole could elevate stresses threefold in 
tension [7]. SEM of the fracture surface revealed brittle mechanisms 
consistent with an overload event with no evidence of fatigue crack 
propagation.  
DISCUSSION 
 Glenoid component failure is the primary reason for revision in 
RSA [1-2]. However, several cases have shown humeral complications 
with severe consequences including fractures [3]. Unscrewing and 
disassembly of modular humeral stems have been noted in a small 
number of cases [3,6]. To our knowledge, this is the first study in 
which unscrewing of the metaphysis-diaphysis junction was coupled 
with fracture of the component during an overload event.  
 The extensive degree of metallosis observed suggests that even 
minimal metal debris from fretting wear can widely impact 
periprosthetic tissue. Optical analysis revealed that fracture occurred at 
the site of a through-hole in the metaphyseal screw. Composite beam 
analysis confirmed an increase in stress due to a lack of proximal bone 
support in a fully intact stem. This internal implant stress was 
increased by the effects of unscrewing on  the geometry and moment 
of inertia. All three of these factors including the through-hole, the 
lack of proximal bone support and the unscrewing mechanism 
contributed to the weaker interface which predisposed the implant to 
failure during an overload event such as the golf swing. Observations 
during retrieval and optical analysis provided evidence for the failure 
mechanism presented in Figure 4.  
 This case demonstrates that both clinical and design 
considerations contributed to a premature failure of a modular RSA. 
Previous literature has suggested several clinical measures to prevent 
unscrewing of modular implants in vivo, including the use of an 
allograft composite or retentive humeral cup to reduce torsional 
instability in patients with proximal humeral bone deficiency [4,6]. 
While clinical actions can prevent unscrewing, our work shows that 
designs that include stress concentrations can significantly reduce 
mechanical integrity, especially during daily patient activity.   
REFERENCES 
[1] Smithers, C.J., Young, A.A., Walch, G., 2011, “Reverse shoulder 
arthroplasty,” Curr. Rev. Musculoskelet. Med., 4, 183-190.  
[2] Seebauer, L., 2007, “Total Reverse Shoulder Arthroplasty: 
European Lessons and Future Trends,” Am. J. Orthop., 36, pp. 22-28. 
[3] Roberts, C. C., Ekelund, A. L., Renfree, K. J., Liu, P. T., Chew, F. 
S., 2007, “Radiologic Assessment of Reverse Shoulder Arthroplasty,” 
RadioGraphics, 27, pp. 223-235. 
[4] Cuff, D., Levy, J.C., Gutierrez, S., Frankle, M.A., 2011, “Torsional 
stability of modular and non-modular reverse shoulder humeral 
components in a proximal humeral bone loss model,” J. Shoulder Elb. 
Surg., 20, 646-651. 
[5] Matsen, F.A. III, Boileau, P., Walch, G., Gerber, G., Bicknell, R.T. 
“The Reverse Total Shoulder Arthroplasty,” J. Bone Joint Surg. Am., 
89, pp. 660-667. 
[6] De Wilde, L., and Walch, G., 2006, “Humeral prosthetic failure of 
reversed total shoulder arthroplasty: A report of three cases,” J 
Shoulder Elb. Surg., 15(2), pp. 260-264. 
[7] Walter, P., 1997, Peterson’s Stress Concentration Factors. 2nd 
Edition. John Wiley & Sons.  
[8] Tjong, S.C., 2009, Advances in Bioemdical Sciences and 
Engineering, Bentham Science Publisher, p.107. 
 
 
Figure 1 | The retrieved TSR, in which fracture occurred at the 
metaphyseal-diaphysis screw junction (circled). 
Table 1 | Values used in composite beam analysis. 
Material E (GPa) 
Diameter 
(mm) 
Cortical Bone 16 [8] 35 
Diaphysis - Phynox (ISO 5832-4) 204 14 
Metaphysis - Alacrite (ISO 5832-7) 248 8 
 
Figure 2 | Cross sectional geometry used in composite beam analysis. 
 
Figure 3 | Illustration of metaphyseal component. The fracture (dotted 
line) occurred at a through-hole incorporated in the screw design. The 
through-hole was filled with a nylon plug.  
 
 
Figure 4 | Optical analysis suggests the following failure mechanisms: 
unscrewing, fretting wear (metallosis), and fracture due to overload. 
Downloaded From: https://proceedings.asmedigitalcollection.asme.org on 07/01/2019 Terms of Use: http://www.asme.org/about-asme/terms-of-use


Design Analysis: Modular Reverse Total Shoulder Prosthetic Failure Without Proximal Bone Support

http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=977A93BD1DCB60F247B5C1AF34E577D6?doi=10.1.1.1045.208&rep=rep1&type=pdf

Design Analysis: Modular Reverse Total Shoulder Prosthetic Failure Without Proximal Bone Support

Abstract

Similar works

Full text

Available Versions

CiteSeerX