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Glenoid loosening in response to dynamic multi-axis eccentric loading: a comparison between keeled and pegged designs with an equivalent radial mismatch.

Abstract

Glenoid loosening is a common failure mode observed in total shoulder arthroplasty. In an effort to isolate the affect of differing fixation techniques on loosening, an edge displacement test was conducted using two, pear-shaped, UHMWPE glenoid designs: one keel and one peg, each having a glenohumeral radial mismatch of 4.3 mm. The susceptibility of each design to loosening was established by quantifiably comparing the maximum glenoid edge displacement before and after 100,000 cycles of eccentric loading by the humeral head along both the superoinferior (SI) and anteroposterior (AP) glenoid axes. Regardless of the axes tested, the results of this study indicate that no discernable difference in edge displacement (distraction and compression) occurred before or after cyclic, eccentric loading for either the keeled or pegged glenoid designs. Additionally, each keel and peg glenoid remained firmly fixed after testing, suggesting that either fixation technique provides sufficient resistance to edge displacement.

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Glenoid loosening is a common failure mode observed in total shoulder arthroplasty. The incidence of loosening is controversial: having been reported to be as low as 0% to 12.5% (1-4) and as high as 30% to 96% (5-15) (as determined by the presence of radiolucent lines, which may or may not correlate with loosening). The current consensus is that glenoid loosening is caused by a mechanism known as the rocking-horse phenomenon. This phenomenon results from cyclic, eccentric loading of the humeral head on the glenoid. A torque is produced about the fixation surface that induces tensile stresses at the implant-cement and bone-cement interfaces. Repetitive eccentric loading may ultimately lead to glenoid failure by disassociation.

Biomechanical studies have demonstrated that the humeral head translates during normal physiologic motion. (16,17) A study by Karduna (16) demonstrated that the humeral head translates 1.5 mm in the anteroposterior (AP) direction and 1.1 mm in the superoinferior (SI) direction during active motion. A study by Friedman (17) measured humeral head translation from radiographs and determined that the humeral head translates approximately 4 mm in the AP direction during active motion. This motion results from the bony incongruence of the glenoid and humerus and the corresponding congruency of the surrounding soft tissue. When the glenoid is resurfaced with a conforming articular surface, edge loading occurs as a result of the inadequacy of ultra high molecular weight polyethylene (UHMWPE) to mimic the viscoelastic properties of the articular cartilage and labrum.

The current consensus is that edge loading can be minimized with a nonconforming glenoid design. The aforementioned study by Karduna16 compared active and passive joint translations between cadaveric joints (before and after) treatment with total shoulder arthroplasty (TSA). The authors concluded that a glenohumeral radial mismatch of 4 mm best reproduced the active translation of the natural joint during internal-external rotation. Similarly, a study by Harryman (18) demonstrated that a glenohumeral radial mismatch of 4 mm closely emulated the passive glenohumeral motion of the natural joint. A study by Walch (19) compared the clinical outcome Constance scores from 319 total shoulders at a mean follow-up of 53.5 months (range: 24 to 110 months). The authors concluded that prostheses having a glenohumeral radial mismatch from 6 mm to 10 mm were associated with the best clinical results (i.e., most range of motion, less pain, and less incidence of radiolucent lines).

The purpose of this study was to quantify the degree of edge displacement associated with two, pear-shaped, UHMWPE glenoid designs: one keel and one peg, each having a glenohumeral radial mismatch of 4.3 mm. The specific aim was to evaluate the null hypothesis that there is no difference in the magnitude of edge displacement between the two designs when subjected to a cyclic, eccentric load by the humeral head along the SI and AP glenoid axes.

Methods

The glenoid loosening study was conducted using 12 glenoids (6 keel and 6 peg; Fig. 1) as specified by ASTM F2028-02. (20) The study was completed in three testing phases: 1. the subluxation test, 2. the rocking test, and 3. the displacement test. Prior to performing each phase, each glenoid was cemented in a polyurethane bone substitute (#1522-02; Pacific Research Laboratories, Inc.), utilizing a standard implantation (drill, ream, broach, etc.) and cementation technique. After the PMMA cement (Cemex; Tecres, Inc.) cured, each implant was positioned into the biaxial testing apparatus depicted in Figure 2. Due to the implant design and implant/instrument sizing, both keeled and pegged glenoids had a cement mantle thickness between 0.5 mm to 1.5 mm. The biaxial testing apparatus compressed the glenoid against the humeral head using a pneumatic actuator while the humeral head was translated parallel to the glenoid plane (in either the SI direction or AP directions) using a hydraulic testing machine.

[FIGURES 1-2 OMITTED]

Phase 1: The Subluxation Test

The goal of the subluxation test was two fold: to quantify the shear force required for humeral head subluxation on the glenoid in both the SI and the AP directions (i.e., the subluxation load) and to quantify the magnitude of humeral head translation on the glenoid prior to humeral head subluxation in both the SI and AP directions (i.e., the subluxation translation), where the subluxation load is defined as the maximum resistive force at the glenoid surface that opposes motion of the humeral head and the subluxation translation is defined as the distance from the origin of the glenoid to the location where the peak subluxation load occurs.

To determine the subluxation load and subluxation translation in the SI directions, the hydraulic testing machine applies a (shear) force parallel to the glenoid plane to translate the humeral head in the SI direction at a rate of 50 mm/min until head subluxation occurs. This test was performed on three keeled glenoids; the pegged glenoid was not tested because it has an identical articular surface geometry. The maximum force required to sublux the head and the total distance in which the head traveled prior to subluxation was recorded in each direction. This procedure was repeated to determine the subluxation load and subluxation translation in the AP direction with one exception: the humeral head was translated parallel to the glenoid plane in the AP direction.

Phase 2: The Rocking Test

The goal of the rocking test was to simulate the "rocking horse" phenomenon in order to evaluate the resistance of each glenoid design to loosening. This dynamic test was performed on six keeled and six pegged glenoids in the SI and AP directions (three keeled and three pegged glenoids were tested in each direction) while in a water-enclosed chamber heated to 37[degrees] C. A 750 N compressive load was applied to each glenoid/bone block as the humeral head was cyclically displaced to 90% of the subluxation distance (as determined from the subluxation test) at 2 Hz for 100,000 cycles. By way of comparison, this cyclic load represents approximately 25 high-load activities per day (such as getting out of a chair) for 10 years. (22) Disassociation was assumed if the axial translation decreased suddenly while performing the test (this observation would indicate a tilt of the glenoid and therefore the onset of loosening).

Phase 3: The Displacement Test

The goal of the displacement test was to quantify glenoid edge displacement in both the SI and AP directions before and after cyclic edge loading. This test was performed on the six keeled and six pegged glenoids used in the rocking test. A 750 N force compressed each glenoid/bone block against the humeral head when the humeral head was in each of the following three positions: 1. when the humeral head and glenoid were aligned at their origins, 2. when the humeral head was positioned at 90% of its subluxation distance (relative to the glenoid) in both the superior and inferior directions, and 3. when the humeral head was positioned at 90% of its subluxation distance (relative to the glenoid) in both the anterior and posterior directions.

The magnitude of edge displacement was quantified using a traveling microscope (magnification of 60X). The glenoid edge displacement was measured as the deviation from the bone block to the edge of the glenoid when the humeral head was placed at each of the three aforementioned positions. As previously noted, these measurements were made in each direction, both before and after cyclic edge loading, for both glenoid designs. Finally, each test component was evaluated using a 10X magnification to document any changes in the test samples and any other pertinent observations. The microscope had a linear accuracy of [+ or -] 0.5 mm.

The results of each test are interpreted and presented according to the methodology presented by Anglin. (21,22) Anglin defined compression as the collective movement of the glenoid and bone substitute on the loaded side, relative to the centrally loaded condition, and distraction as the collective movement of the glenoid and bone substitute on the unloaded side, relative to the centrally loaded position. The total SI subluxation translation is presented as the average translation in both the superior and inferior directions. Similarly, the total AP subluxation translation is presented as the average translation in both the anterior and posterior directions. Finally, the force ratio (effectively describing the degree of glenoid constraint) is quantified by a ratio of the subluxation load and the applied compressive force.

Results

The results of the subluxation and displacement tests are presented in Tables 1 and 2, respectively. The results of the displacement test demonstrate that little-to-no edge displacement occurs for either the keeled or pegged glenoid following 100,000 cycles of eccentric loading in the SI and AP directions. The maximum average difference between pre- and post-fatigue distraction is 0.32 mm, a negligible amount considering the accuracy of the technique is [+ or -] 0.5 mm. (It should be noted that the "negative differences" between the pre-fatigue and post-fatigue measurements are a result of UHMWPE cold flow.) These findings are corroborated by the observation that no glenoid (keeled or pegged) disassociated following cyclic loading.

Discussion

The results of this study are validated by the results of two different peer-reviewed publications by Anglin, (21,22) each utilizing a similar testing methodology. The first study (21) reported force ratios and subluxation translations (in both the SI and AP directions) of six different glenoid designs having a variety of conformities (radial mismatches varying from 0 mm to 30 mm). The reported force ratios vary from 0.45 to 0.95 in the superior direction, 0.3 to 0.95 in the inferior direction, 0.25 to 0.7 in the anterior direction, and 0.3 to 0.7 in the posterior direction. These reported measurements closely match those presented in this study: 0.93 in the superior direction, 0.77 in the inferior direction, 0.53 in the anterior direction, and 0.50 in the posterior direction. It should be noted that the two designs reported by Anglin that have the closest radial mismatch (approximately 3.5 mm) as those components tested in this study (4.3 mm) had comparable force ratios in each plane: approximately 0.7 in the superior direction, 0.8 in the inferior direction, 0.5 in the anterior direction, and 0.5 in the posterior direction.

Additionally, Anglin (21) reported subluxation translations that varied from 1 mm to 14 mm in the superior direction, 2 mm to 8 mm in the inferior direction, 2 mm to 8 mm in the anterior direction, and 1 mm to 8 mm in the posterior direction. These reported measurements closely match those presented in this study: approximately 5.0 mm in the superior direction, approximately 3.5 mm in the inferior direction, approximately 3.3 mm in the anterior direction, and approximately 3.0 mm in the posterior direction. It should be noted that the two designs that have the closest radial mismatch as those components tested in this study had comparable translations in each direction: approximately 4 mm in the superior direction, approximately 4 mm in the inferior direction, approximately 3.5 mm in the anterior direction, and approximately 4 mm in the posterior direction. Of note, these translations are very similar to those reported by Friedman to occur naturally in the glenohumeral joint during active joint motion. (17)

The second study (22) by Anglin, using a similar testing methodology, reported edge distraction and compression displacements with the aforementioned 6 glenoids before and after SI cyclic edge loading. Pre-fatigue distraction average values ranged from 0.05 mm to 0.2 mm and post-fatigue distraction average values 0.05 mm to 0.25 mm. Both of which are very similar to those observed in this study: pre-distraction: 0.07 mm to 0.17 mm; and post-distraction: 0.07 mm to 0.14 mm. Similarly, pre-fatigue compression average values ranged from 0.3 mm to 0.5 mm and post-fatigue compression average values ranged from 0.3 mm to 0.6 mm. Once again, very similar to those values observed in this study: pre-compression: 0.09 mm to 0.16 mm; and post-compression: 0.22 mm to 0.44 mm. Finally, the difference in average distraction values between pre-fatigue and post-fatigue ranged from 0.0 mm to 0.2 mm, also very similar to the range of values reported in this study: 0.0 mm to 0.04 mm.

There are three major limitations to this study, each of these limitations contributed to a higher than desired standard deviation in the reported data. First, only three glenoids were tested in each plane for each of the two designs. Second, the traveling microscope used to detect edge displacement only had an accuracy of [+ or -] 0.5 mm (due primarily to UHMWPE deformations at the rim as a result of the cyclic edge loading) and this value is approximately the same magnitude as that of the largest recorded edge displacement. The gage dials used in the aforementioned studies by Anglin (21,22) had a reported accuracy of 0.05 mm; these dials clearly provided a more sophisticated measuring technique. And finally, the use of a polyurethane bone substitute instead of the more clinically relevant cadaveric bone can be considered a limitation. The polyurethane bone substitute was chosen because it has consistent and uniform mechanical properties similar to that of glenoid cancellous bone (a density of 0.24 g/cc, compressive tensile strength of 8.8 MPa, and a compressive modulus equal to 260 MPa, as graded according to ASTM F-1839). (23,24) This consistency in material properties is paramount for a one-to-one comparison such as the one performed in this study. The mechanical properties of cadaveric bone would be nonuniform from sample to sample.

However, the accuracy of the employed technique was sufficient to demonstrate that the observed edge displacements were minimal and comparable to that of published values for six different clinically successful glenoid designs. For this reason, the results of this study suggest that the fixation provided by both the keeled and pegged glenoid designs are sufficient to resist loosening via cyclic, eccentric loading. It is therefore concluded that both the keeled and pegged glenoid designs provide an acceptable level of resistance to glenoid loosening, the primary failure mode of total shoulder arthroplasty.

To the authors' knowledge, no study has characterized glenoid edge displacements before and after cyclic, eccentric loading in the AP directions, or quantifiably compared its effect on two equivalent keeled and pegged glenoid designs (having an equivalent shape, size, and radial mismatch). Therefore, this study contributes new information to the body of knowledge relating to glenoid loosening. This information is valuable because edge loading could occur more commonly (in the absence of superior humeral head migration) in the AP directions (via internal/external rotation) than the SI directions (via abduction/adduction) during an individual's activities of daily living.

Conclusion

Regardless of the direction tested, the results of this study demonstrate that no discernable difference in edge displacement (distraction and compression) occurred before or after cyclic, eccentric loading for either the keeled or pegged glenoid designs. Additionally, the average subluxation loads, translations, and force ratios quantified in the subluxation test fall within the range of those reported by Anglin21 for each plane of motion. Similarly, the average pre-fatigue and post-fatigue, compression and distraction values quantified in the rocking and displacement tests fall within the range of those reported by Anglin (22) for SI cyclic edge loading.

With respect to glenoid loosening, since each keeled and pegged glenoid remained firmly fixed after testing, the results of this study suggest that either fixation technique provides a sufficient resistance to edge displacement, for a radial mismatch equal to 4.3 mm. For this reason, we fail to reject the null hypothesis and conclude that there is no difference in the magnitude of edge displacement between the two fixation designs when subjected to a cyclic, eccentric load in the SI and AP directions.

References

(1.) Brems J, et al: The glenoid component in total shoulder arthroplasty. J Shoulder Elbow Surg. 1993;2:47-54, 1993.

(2.) Mestdagh H, et al: Intra- and postoperative complications of shoulder arthroplasty. In: Shoulder Arthroplasty. Berlin: Springer-Verlag, 1999, pp. 163-167.

(3.) Rodosky MW, et al: Indication for glenoid resurfacing in shoulder arthroplasty. J Shoulder Elbow Surg. 1996;5:231-48.

(4.) Wirth MA, Rockwood CA Jr: Complications of shoulder arthroplasty. Clin Orthop Relat Res. 1994;307:47-69.

(5.) Bade H, et al: Long-term results of Neer total shoulder replacement. In: Surgery of the Shoulder. St. Louis: Mosby, 1984, pp. 249-252.

(6.) Barrett WP, Franklin JL, Jackins SE, Wyss CR, Matsen FA 3rd: Total shoulder arthroplasty. J Bone Joint Surg Am. 1987;69:865-72.

(7.) Boileau P, et al: Neer shoulder prosthesis: results related to etiology. Rev Rheum. 1994;61:539-47.

(8.) Boyd AD, et al: Glenoid resurfacing in shoulder arthroplasty. In: Arthroplasty of the Shoulder. New York: Thieme, 1994, pp. 306-16.

(9.) Brenner BC, et al: Survivorship of unconstrained total shoulder arthroplasty. J Bone Joint Surg. 1989;71:1289-96.

(10.) Cofield RH: Total shoulder arthroplasty with the Neer prosthesis. J Bone Joint Surg. 1984;64:319-37.

(11.) Hawkins RJ, et al: Total shoulder arthroplasty. Clin Orthop. 1989;242:188-94.

(12.) Kempf JF, et al: Results of shoulder arthroplasty in primary glenohumeral osteoarthritis. In: Shoulder Arthroplasty. Berlin: Springer-Verlag, 1999, pp. 203-210.

(13.) Mole D, et al: Cemented glenoid component: results in osteoarthritis and rheumatoid arthritis. In: Shoulder Arthroplasty. Berlin: Springer-Verlag, 1999, pp. 163-167.

(14.) Neer CS, et al: Recent experience in total shoulder replacement. J Bone Joint Surg. 1982;64:319-37.

(15.) Wilde AH, et al: Experience with the Neer total shoulder replacement. In: Surgery of the Shoulder. St. Louis: Mosby, 1984, pp. 224-228.

(16.) Karduna AR, et al: Glenohumeral joint translations before and after total shoulder arthroplasty: a study in cadavera. J Bone Joint Surg Am. 1997;79(8):1166-74.

(17.) Friedman RJ: Glenohumeral translation after total shoulder arthroplasty. J Shoulder Elbow Surg. 1992;1:312-16.

(18.) Harryman DT, et al: Effect of articular conformity and the size of the humeral head component on laxity and motion after glenohumeral arthroplasty. J Bone Joint Surg Am. 1995;77(4):555-63.

(19.) Walch G, et al: Influence of glenohumeral prosthetic mismatch on glenoid radiolucent lines. J Bone Joint Surg Am. 2002;84(12):2186-91.

(20.) ASTM F 2028-02. Standard test methods for the dynamic evaluation of glenoid loosening or disassociation. Annual Book of ASTM Standards. Volume 13.01, pp. 1083-1088, 2004.

(21.) Anglin C: Shoulder prosthesis subluxation: theory and experiment. J Shoulder Elbow Surg. 2000;9(2):104-14.

(22.) Anglin C, et al: Mechanical testing of shoulder prostheses and recommendations for glenoid design. J Shoulder Elbow Surg. 2000;9(4):323-31.

(23.) ASTM F 1839-00. Standard specification of rigid polyurethane foam for use as a standard material for testing orthopaedic devices and instruments. Annual Book of ASTM Standards. Vol. 13.01, pp. 916-921, 2004.

(24.) Anglin C, et al: Glenoid cancellous bone strength and modulus. J Biomech. 1999;32:1091-7.

C. Roche, M.S., and L. Angibaud, B.S., are at Exactech, Gainesville, Florida. P. H. Flurin, M.D., is at the Bordeaux-Merignac Sports Clinic, France. T. Wright, M.D., is in the Department of Orthopaedic Surgery, University of Florida, Gainsville, Florida. Joseph Zuckerman, M.D., is in the NYU-Hospital for Joint Diseases Department of Orthopaedic Surgery, New York, New York.

Correspondence: C. Roche, M.S., Product Development Manager, Exactech, Inc., 2320 NW 66th Court, Gainesville, Florida 32653.
Table 1 Subluxation Test Results: Maximum Shear Forces and
Translations

Subluxation Parameter Avg [+ or -] STD

Superior Subluxation Load 695.7 [+ or -] 55.5 N
Superior Subluxation Translation 5.49 [+ or -] 1.06 mm
Inferior Subluxation Load 575.0 [+ or -] 54.1 N
Inferior Subluxation Translation 3.94 [+ or -] 0.55 mm
Anterior Subluxation Load 395.3 [+ or -] 44.1 N
Anterior Subluxation Translation 3.72 [+ or -] 0.17 mm
Posterior Subluxation Load 375.0 [+ or -] 46.4 N
Posterior Subluxation Translation 3.42 [+ or -] 0.32 mm
Total SI Subluxation Translation 4.71 [+ or -] 0.67 mm
Total AP Subluxation Translation 3.57 [+ or -] 0.23 mm

Subluxation Parameter 90% Avg

Superior Subluxation Load 626.10 N
Superior Subluxation Translation 4.94 mm
Inferior Subluxation Load 517.50 N
Inferior Subluxation Translation 3.54 mm
Anterior Subluxation Load 355.8 N
Anterior Subluxation Translation 3.35 mm
Posterior Subluxation Load 337.5 N
Posterior Subluxation Translation 3.08 mm
Total SI Subluxation Translation NA
Total AP Subluxation Translation NA

Subluxation Parameter Force Ratio

Superior Subluxation Load 0.93
Superior Subluxation Translation NA
Inferior Subluxation Load 0.77
Inferior Subluxation Translation NA
Anterior Subluxation Load 0.53
Anterior Subluxation Translation NA
Posterior Subluxation Load 0.50
Posterior Subluxation Translation NA
Total SI Subluxation Translation NA
Total AP Subluxation Translation NA

Table 2 Displacement Test Results: Average Keeled and Pegged Glenoid
Compression and Distraction, Before and After Superoinferior
and Anteroposterior Loading

Glenoid Edge Avg [+ or -] STD: Avg [+ or -] STD:
Displacement Pre-Fatigue Post-Fatigue
Direction Compression Compression

Keel Glenoid (SI) 0.16 [+ or -] 0.44 mm 0.09 [+ or -] 0.59 mm
Peg Glenoid (SI) 0.44 [+ or -] 0.34 mm 0.22 [+ or -] 0.83 mm
Keel Glenoid (AP) 0.44 [+ or -] 0.30 mm 0.34 [+ or -] 0.91 mm
Pee Glenoid (AP) 0.44 [+ or -] 0.24 mm 0.32 [+ or -] 0.31 mm

Glenoid Edge Avg [+ or -] STD: Avg [+ or -] STD:
Displacement Pre-Fatigue Post-Fatigue
Direction Distraction Distraction

Keel Glenoid (SI) 0.17 [+ or -] 0.55 mm 0.10 [+ or -] 0.49 mm
Peg Glenoid (SI) 0.07 [+ or -] 0.12 mm 0.14 [+ or -] 0.47 mm
Keel Glenoid (AP) 0.09 [+ or -] 0.18 mm 0.05 [+ or -] 0.57 mm
Pee Glenoid (AP) 0.41 [+ or -] 0.51 mm 0.31 [+ or -] 0.44 mm
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Author:Roche, C.; Angibaud, L.; Flurin, P.H.; Wright, T.; Zuckerman, Joseph
Publication:Bulletin of the NYU Hospital for Joint Diseases
Date:Jan 1, 2006
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