Glenoid Loosening in Response to Dynamic Multi-Axis Eccentric Loading: a Comparison Between Keeled and Pegged Designs with an Equivalent Radial Mismatch

Roche, C and Angibaud, L (Exactech, Gainesville, FL)
Flurin, PH (Bordeaux-Merignac Sports Clinic, FR)
Wright, T (Shands Hosp., Gainesville, FL)
Zuckerman, J (Hosp. for Joint Diseases, NY, NY)

3rd International
Conference on Shoulder
Arthroplasty
Paris • 2005

Abstract

A common failure mode observed in total shoulder arthroplasty is loosening of the glenoid. 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 4.3mm. The susceptibility of each design to loosening was established by quantifiably comparing the maximum glenoid edge displacement before and after cyclic, eccentric loading by the humeral head along both the superior/inferior (S/I) and anterior/posterior (A/P) 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 and after cyclic, eccentric loading for either the keeled or pegged glenoid designs. Additionally, each keel and peg glenoid remained firmly fixed after testing indicating that either fixation technique provides a sufficient resistance to edge displacement, for a radial mismatch = 4.3mm.

Introduction

A common failure mode observed in total shoulder arthroplasty is loosening of the glenoid. The incidence of loosening is controversial: having been reported to be as low as 0-12.5%^1-4 and as high as 30-96%^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; effectively, inducing a torque about the fixation surface thereby increasing 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 Karduna¹⁶ demonstrated that the humeral head translates 1.5 mm in the anterior/posterior (A/P) direction and 1.1 mm in the superior/inferior (S/I) direction during active motion. A study by Friedman¹⁷ measured humeral head translation from radiographs and determined that the humeral head translates ~4mm in the A/P 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 results due to the inadequacy of UHMWPE to mimic the visco-elastic 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 & passive joint translations between cadaveric joints (before & after) treatment with 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 Harryman¹⁸ demonstrated that a glenohumeral radial mismatch of 4 mm closely emulated the passive glenohumeral motion of the natural joint. A study by Walch¹⁹ compared the clinical outcome constance scores from 319 total shoulders at a mean follow-up of 53.5 months (range: 24-110 months). The authors concluded that a prostheses having a glenohumeral radial mismatch from 6-10mm were associated with the best clinical results (i.e. most range of motion, less pain, & less incidence of radiolucent lines).

The purpose of this study is 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 4.3mm. The specific aim is 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 S/I and A/P glenoid axes.

Methods

The glenoid loosening study was conducted using 12 glenoids (6 keel and 6 peg; Figure 1) as specified by ASTM F2028-00.²⁰ 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 (Last-a-Foam FR6720), utilizing a standard implantation (drill, broach, burr, etc…) and cementation technique. After the PMMA cement (Cemex) cured, each implant was positioned into the biaxial testing apparatus depicted in Figure 2. 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 S/I direction or A/P directions) using a hydraulic testing machine.

Figure 1. Keeled and Pegged Glenoid		Figure 2. Biaxial Testing Apparatus20

Phase 1: The Subluxation Test

The goal of the subluxation test was two fold: 1) to quantify the shear force required for humeral head subluxation on the glenoid in both the S/I & the A/P directions (i.e. the subluxation load) & 2) to quantify the magnitude of humeral head translation on the glenoid prior to humeral head subluxation in both the S/I & A/P 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/subluxation translation in the S/I directions, the hydraulic testing machine applies a (shear) force parallel to the glenoid plane to translate the humeral head in the S/I direction at a rate of 50 mm/min until head subluxation occurs. 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/subluxation translation in the A/P direction with one exception: the humeral head was translated parallel to the glenoid plane in the A/P 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 glenoid designs (using the aforementioned biaxial apparatus, Figure 2) in a water-enclosed chamber, heated to 37º 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 ~25 high-load activities (such as getting out of a chair) a day for 10 years.²² This test was performed in both the S/I & A/P directions, for both the pegged & keeled glenoids. Disassociation was assumed if the axial translation decreases 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 S/I & A/P 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 & inferior directions, & 3) when the humeral head was positioned at 90% of its subluxation distance (relative to the glenoid) in both the anterior & 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 3 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 microscope to document any changes to the test samples and any other pertinent observations. The microscope was associated with a linear accuracy of ± 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 S/I subluxation translation is presented as the average translation in both the superior & inferior directions. Similarly, the total A/P subluxation translation is presented as the average translation in both the anterior & 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.

Table 1. Subluxation Test Results: Maximum Shear Forces and Translations.

Table 2. Displacement Test Results: Avg. Keeled and Pegged Glenoid Compression and
Distraction, Before and After S/I and A/P Loading.

Results

The results of this study clearly demonstrate that little-tono edge displacement occurs for either the keeled or pegged glenoid following 100,000 cycles of eccentric loading in the S/I and A/P 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 ± 0.5 mm. (It should be noted that the "negative differences" between the pre 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 study²¹ reports on force ratios & subluxation translations (in both the S/I & A/P directions) of six different glenoid designs having a variety of conformities (radial mismatches varying from 0-30 mm). The reported force ratios vary from 0.45-0.95 in the superior direction, 0.3-0.95 in the inferior direction, 0.25-0.7 in the anterior direction, & 0.3-0.7 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, & 0.50 in the posterior direction. It should be noted that the two designs reported by Anglin which have the closest radial mismatch (~3.5 mm) as those components tested in this study (4.3 mm) had comparable force ratios in each plane: ~0.7 in the superior direction, 0.8 in the inferior direction, 0.5 in the anterior direction, & 0.5 in the posterior direction.

Additionally, Anglin²¹ reports the subluxation translations vary from 1-14 mm in the superior direction, 2-8 mm in the inferior direction, 2-8 mm in the anterior direction, & 1-8 mm in the posterior direction. These reported measurements closely match those presented in this study: ~5.0 mm in the superior direction, ~3.5 mm in the inferior direction, ~3.3 mm in the anterior direction, & ~3.0 mm in the posterior direction. It should be noted that the two designs which have the closest radial mismatch as those components tested in this study had comparable translations in each direction: ~4 mm in the superior direction, ~4 mm in the inferior direction, ~3.5 mm in the anterior direction, & ~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 study²² by Anglin, using a similar testing methodology, reports edge distraction & compression displacements with the aforementioned 6 glenoids before & after S/I cyclic edge loading. Pre-fatigue distraction average values ranged from 0.05-0.2 mm & post-fatigue distraction average values 0.05-0.25 mm. Both of which are very similar to those observed in this study: pre-distraction: 0.07-0.17 mm & post-distraction: 0.07-0.14 mm. Similarly, pre-fatigue compression average values ranged from 0.3-0.5 mm & post-fatigue compression average values ranged from 0.3-0.6 mm. Once again, very similar to those values observed in this study: precompression: 0.09-0.16 mm & post-compression: 0.22-0.44 mm. Finally, the difference in average distraction values between pre- & post-fatigue ranged from 0.0-0.2 mm, also very similar to the range of values reported in this study: 0.0-0.04 mm.

There are two major limitations to this study, each of these limitations contributed to a higher than desired standard deviation in the reported data. These limitations are as follows: 1) only three glenoids were tested in each plane for each of the two designs & 2) the traveling microscope used to detect edge displacement only had an accuracy of ±0.5 mm (due primarily to UHMWPE deformations at the rim as a result of the cyclic edge loading) - this value is approximately the same magnitude as that of the largest recorded edge displacement. The gage dials used in the aforementioned Anglin studies^21,22 had a reported accuracy of 0.05 mm; these dials clearly provided a more sophisticated measuring technique.

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 clearly demonstrate that the fixation provided by both the keeled and pegged glenoid design is sufficient to resist loosening via eccentric, cyclic loading. It is therefore concluded that both the keeled & 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 A/P directions…or quantifiably compared its effect on two equivalent keeled & pegged glenoid designs (having an equivalent 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 A/P directions (via internal/external rotation) than the S/I directions (via abduction/adduction) during an individual's activities of daily living.

Conclusion

In conclusion, regardless of the axes tested, the results of this study indicate that no discernable difference in edge displacement (distraction and compression) occurred before and after cyclic, eccentric loading for either the pegged or keeled glenoid designs. Additionally, the average subluxation loads, translations, and force ratios quantified in the subluxation test fall within the range of those reported by Anglin,²¹ for each plane of motion. Similarly, the average pre- 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 S/I cyclic edge loading.

With respect to glenoid loosening, the results of this study indicate that either fixation technique provides a sufficient resistance to edge displacement, for a radial mismatch = 4.3mm. 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 designs when subjected to a cyclic, eccentric load in the S/I and A/P directions.

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Production of this paper sponsored by:
Exactech, Inc., General Research Fund