Optimization of Cemented Glenoid Peg Geometry

Bulletin of the Hospital for Joint Diseases 2015;73(Suppl 1):S33-6

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Optimization of Cemented Glenoid Peg Geometry A Comparison of Resistance to Axial Distraction Lisa Becks, M.S., Corey Gaydos, B.S., Nicholas Stroud, M.S., and Christopher P. Roche, M.S., M.B.A.

Abstract Introduction: Glenoid loosening is one of the most common complications of anatomic total shoulder arthroplasty (aTSA). Numerous glenoid pegged designs exist within the market place; however, little effort has been made to optimize peg geometry, and as a result, there is no consensus regarding the superiority of one design over another. The aim of this study was to determine the impact of peg design on the fixation strength by comparing the force and displacement associated with five different geometries of cemented glenoid components when each is axially displaced from two different densities of polyurethane bone substitute substrates. Methods: An axial pull-out test was conducted on five different cemented peg geometries in both low- and highdensity polyurethane bone-substitute blocks. All substrates were prepared utilizing a drill, which created a 7.3 mm diameter hole to a depth of 26.8 mm. Cemex® brand bone cement was prepared and used to cement all pegs. After cementation of each peg, an electromechanical load frame applied a linear ramp displacement of 10 mm/minute axially to each peg while the polyurethane block was fully constrained. Load and displacement of each peg was sampled at 100 Hz until failure and axial distraction of each peg where the peak pull-out force and associated displacement were recorded. The average load to failure and associated displacement for each peg geometry were compared utilizing the Student’s t-test where a p-value < 0.05 determined significance.

Lisa Becks, M.S., Corey Gaydos, B.S., Nicholas Stroud, M.S., and Christopher P. Roche, M.S., M.B.A., Exactech, Inc., Gainesville, Florida. Correspondence: Christopher P. Roche, M.S., M.B.A., Exactech, Inc., 2320 NW 66th Court, Gainesville, Florida 32653; chris.roche@ exac.com.

Results: Cemented peg design #3 was associated with the greatest axial load to failure (675.3 N ± 18.8 N in low density and 707.3 N ± 11.7 N in high density) for both densities of bone-substitute blocks. Peg designs #5 and #2 were associated with the next highest axial loads to failure in both low and high density blocks. Finally, peg designs #1 and #4 were associated with the lowest axial loads to failure in both low and high density blocks. Only design #3 had a statistically significant difference between peak pull-out forces between the low- and high-density bone substitute blocks, as compared to all other designs. Conclusions: The results of this study demonstrate that glenoid peg geometry can significantly influence the resistance to axial distraction where the continuous threaded geometry exemplified by peg design #3 demonstrated significantly superior cemented fixation relative to the other peg designs. It can be concluded that overall macrostructure and design of the peg itself plays a key role in pull-out force; however, performance in a clinical setting is required to confirm these biomechanical results.

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lenoid loosening is one of the most common complications of total shoulder arthroplasty and is considered to be the limiting long-term failure mode. The causes of loosening are multi-factorial and include off-center or rim loading, called “rocking horse phenomenon,” constrained prosthetic systems that induce too great a torque on the fixation surfaces, poor implantation technique, use in eroded or compromised scapula, and poor cementation technique.1-3 There are two typical designs of glenoid components, namely keel or peg. Keel designs, as compared to peg designs, require more bone resection and also more cement and have been associated with increased rotation and translation,4 as well as increased radiolucent lines.5 Peg glenoids remove less bone and typically require less cement. Numerous glenoid

Becks L, Gaydos C, Stroud N, Roche CP. Optimization of cemented glenoid peg geometry: a comparison of resistance to axial distraction. Bull Hosp Jt Dis. 2015;73(Suppl 1):S33-6.

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Figure 1 Five different types of peg geometries tested.

pegged designs exist within the market place; however, little effort has been made to optimize peg geometry, and as a result, there is no consensus regarding the superiority of one design over another. The aim of this study is to determine the impact of peg design on the fixation strength by comparing the force and displacement associated with five different geometries of cemented glenoid components when each is axially displaced from two different densities of polyurethane bone substitute substrates.

Materials and Methods Five unique center peg geometries (Fig. 1) were designed in a 3D computer modeling software (Unigraphics NX, Siemens, Inc.). Each peg design was manufactured from GUR 1050 Ultra High Molecular Weight Polyethylene (UHMWPE) with an equivalent surface finish in order to test cement fixation by axial distraction. All devices were vacuum packaged and gamma-sterilized to a maximum dosage of 37 kGy. Peg designs #1 and #4 were intended to simulate the geometry of FDA-cleared cemented devices with more than 10 years of clinical experience, whereas peg designs #2, #3, and #5 were novel and were intended to maximize cement mantle thickness and uniformity. All pegs were 13.5 mm in length except for design #1, which was only 12 mm long. Similarly, all designs utilized a 7° taper except for design #4, which was cylindrical. Eight samples of each peg geometry were assembled in both low- or high-density polyurethane bone substitute (15 pcf and 30 pcf, respectively) blocks [conforming to ASTM F1839-08(2012) to simulate poor and good quality bone] for a total of 80 tested samples. Both the low and high density blocks were prepared utilizing a drill creating a 7.3 mm diameter hole to a depth of 26.8 mm. Cemex® brand bone cement (Tecres, Inc., Verona, Italy) was used to cement all pegs. After cementation of each peg to the substrate, an electromechanical load frame applied a linear ramp displacement of 10 mm/minute axially to each peg while the polyurethane block was fully constrained (Fig. 2). To ensure that each test sample was loaded axially, a universal joint was used to link the sample to the test frame. Load and displacement of each peg were sampled at 100 Hz until failure or axial distraction of each peg. The average load to failure and associated displacement for each peg geometry were recorded and compared utiliz-

ing the Student’s unpaired, two-tailed t-test, where p-value < 0.05 determined significance.

Results The average axial force required to extract each peg design in both the low and high density polyurethane blocks is described in Figure 3. Numerous differences were noted in the peak pull-out forces between designs and substrate densities (Table 1). Peg design #3 was associated with the greatest axial load to failure (675.3 N ± 18.8 N in low density and 707.3 N ± 11.7 N in high density) for both densities of bone-substitute blocks. Peg designs #5 and #2, respectively, were associated with the next highest axial loads to failure in both low and high density blocks. Finally, peg designs #4 and #1 were associated with the lowest axial loads to failure in both low and high density blocks. Failure modalities between peg designs were similar

Figure 2 Photograph of the test setup immediately after completing axial pull out testing of one of the peg design samples.


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Table 1 The Results of the Statistical Analysis are Provided in the Form of P-values Found when Comparing the Mean Peak Pull-Out Force for Each of the Tested Peg Geometries Design 1 Design 1 Design 2

< 0.0001

Design 2

Design 3

Design 4

Design 5

< 0.0001

< 0.0001

0.0002

< 0.0001

< 0.0001

0.0013

0.0027

Design 3

< 0.0001

< 0.0001

Design 4

0.2878

0.0002

Design 5

< 0.0001

0.0120

< 0.0001 < 0.0001 < 0.0001

< 0.0001

High Density

< 0.0001 < 0.0001

Low Density

Table 2 The Results of the Statistical Analysis are Provided in the Form of P-values Found when Comparing the Mean Displacement at Peak Pull-Out Force for Each of the Tested Peg Geometries Design 1 Design 1

Design 2

Design 3

Design 4

Design 5

0.0002

0.9350

0.0010

< 0.0001

0.1150

< 0.0001

0.0510

0.1280

Design 2

0.0004

Design 3

0.1130

0.0010

Design 4

0.2160

0.0070

Design 5

< 0.0001

< 0.0001

0.0790 0.9420 0.3260

High-Density

< 0.0001 0.3690

Low-Density

for the high density blocks (in which the peg disassociated from the cement in all cases) but different for the low density blocks, where in some cases the substrate failed prior to the peg disassociating from the cement. Six of the eight tested pegs of peg design #3 failed in the low density block by the substrate fracturing before the peg disassociated from the cement. It should be noted that only four samples of peg design #4 were able to be evaluated in high-density foam due to poor assembly: two of these samples were able to be removed with the force of gravity, and the other two samples had been deformed during assembly such that they could not be attached to the end of the load frame; thus, these samples were excluded from the results. Plastic

deformation was observed to some extent on all samples during axial extraction. The average displacement to peak pull-out force for each peg geometry is presented in Figure 4. Differences in mean displacement at peak pull-out force were observed between designs and substrate densities, (Table 2) where design #5 was associated with the lowest displacement.

Figure 3 Average peak pull-out force for each test group in low- and high-density bone. The error bars represent one standard deviation.

Figure 4 Average load displacement curves at peak pull-out force for each test group in low- and high-density bone. The error bars represent one standard deviation.

Discussion Given the growth of total shoulder arthroplasty over the last decade, the uniformity of belief that the cemented peg glenoid is the gold-standard, and persistent concerns about the complication of aseptic glenoid loosening as the long-

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term failure mode, it is important that more effort be made to optimize the design of the cemented peg glenoid. Anglin and others have evaluated different design parameters over 15 years ago—but little work has been done since to further refine and optimize the design of the cemented peg glenoid.6 This current study evaluates the impact of two commercially successful cemented center peg designs, each with over 10 years clinical experience and compares the cemented fixation in both low and high density bone blocks to that of three novel peg geometries that facilitates a more uniform and thicker cement mantle over the length of the peg. The average peak pull-out results of this study objectively demonstrated that peg geometry #3 provided superior resistance to axial extraction in both low and high density bone-substitute blocks as compared to the other four designs evaluated; however, the displacement at the peak pull-out forces were minimally different between peg designs. The exact reasons for the superior results of design #3 are unclear; however, we suspect that the thicker helical thread form permitted greater flow of the cement around the peg while the larger outer diameter along the length of the peg created greater cement pressurization. Several recent studies in the literature have demonstrated that greater cement pressurization results in a reduction of the incidence of radiolucent lines clinically.7,8 It may be that a peg geometry, such as utilized in design #3, is able to achieve pressurization without the need for supplemental pressurization instrumentation. This study has several limitations. We utilized a polyurethane bone-substitute block of two different densities to simulate both good and poor quality bone rather than actual cadaveric bone for reasons of cost and concerns of uniformity in bone quality across all test samples. Improvements to this test could also include the use of a composite substrate to simulate cortical and cancellous bone. Additionally, this test strictly quantified the impact of different peg geometries on axial pull-out strength; we fully recognize that this axial-loading methodology is non-physiological and does not simulate the clinical failure mechanism as described by the rocking horse phenomenon.6 However, as translational movements of the humeral head on the glenoid component occur during glenohumeral movement, a greater pull-out strength of the fixation pegs can be beneficial for stability of the component.9 Future work should evaluate the impact of different cement viscosities and also different thicknesses of cement mantle by the use of different diameters of drills on cemented peg fixation. Additionally, as these results objectively demonstrate that the axial resistance of cemented peg design #3 is superior to the other devices tested, future work should evaluate the combined application of this peg geometry on both central and peripheral pegs. Finally, additional testing to qualify this peg geometry is required to confirm sufficient

resistance to the applied torque resulting from eccentric humeral head edge loading by the rocking-horse phenomenon.6

Conclusion The results of this study demonstrate that glenoid peg geometry can significantly influence the resistance to axial distraction, where the continuous threaded geometry exemplified by peg design #3 demonstrated superior cemented fixation relative to the other peg designs tested in this study. It can therefore be concluded that overall macrostructure and design of the peg itself plays a key role in pull-out force of cemented UHMPWE pegs. While the clinical application of this novel peg geometry appears promising based upon the results of this biomechanical study, these laboratory results are not a substitute for clinical performance. Ultimately, long-term clinical follow-up is necessary to demonstrate glenoid design optimization through the reduced incidence of radiolucent lines and aseptic glenoid loosening as a complication. Conflict of Interest Statement Funding for this study was provided by Exactech, Inc. Lisa Becks, M.S., Corey Gaydos, B.S., Nicholas Stroud, M.S., and Christopher P. Roche, M.S., M.B.A., are employees of Exactech, Inc., Gainesville, Florida

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Franklin JL, Barrett WP, Jackins SE, Matsen FA III. Glenoid loosening in total shoulder arthroplasty: association with rotator cuff deficiency. J Arthroplasty. 1988;3(1):39-46. 2. Post M. Constrained arthroplasty of the shoulder. Orthop Clin North Am. 1987 Jul;18(3):455-62. 3. Norris BL, Lachiewicz PF. Modern cement technique and the survivor-ship of total shoulder arthroplasty. Clin Orthop 1996 Jul;(328):76-85. 4. Nuttall D, Haines JF, Trial II. A study of the micromovement of pegged and keeled glenoid components compared using radiostereometric analysis. J Shoulder Elbow Surg 2007 MayJun;16(3 suppl); S65-70. 5. Gartsman GM, Elkousy HA, Warnock KM, et al. Radiographic comparison of pegged and keeled glenoid components. J Shoulder Elbow Surg. 2005 May-Jun;14(3):252-7. 6. Anglin C, Wyss UP, Pichora DR. Mechanical testing of shoulder prostheses and recommendations for glenoid design. J Shoulder Elbow Surg. 2000 Jul-Aug;9(4):323-31. 7. Choi T, Horodyski M, Struk A, et al. Incidence of early radiolucent lines after glenoid component insertion for total shoulder arthroplasty: a radiographic study comparing pressurized and unpressurized cementing techniques. J Shoulder Elbow Surg. 2013 Mar;22(3):403-8. 8. Raiss P, Sowa B, Bruckner T, et al. Pressurisation leads to better cement penetration into the glenoid bone. J Bone Joint Surg Br. 2012 May;94(5):671-7. 9. Nyffeler RW, Anglin C, Sheikh R, Gerber C. Influence of peg design and cement mantle thickness on pull-out strength of glenoid component pegs. J Bone Joint Surg Br. 2003 Jul;85(5):748-52.