Anatomic Validation of the Equinoxe Total Shoulder System

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

Introduction

The underlying premise behind a "third generation" or anatomic shoulder prosthesis is that joint stability is best achieved & maintained after arthroplasty by restoring natural soft tissue tension - and natural soft tissue tension is best restored by implanting a prosthesis that restores the patients proximal humeral anatomy; thereby, reproducing the natural center of rotation. To this end, an anatomic study is conducted (and subsequently compared to the results of several peer-reviewed, published anthropomorphic studies) in an effort to ascertain the natural anatomic variability of the structures composing the glenohumeral joint. By comparing these ranges to the geometric variability provided by the Equinoxe, it can be demonstrated that the Equinoxe provides sufficient variability to enable an anatomic reconstruction for all humeri populations.

A word of caution should be voiced in regards to direct comparisons between these studies - as each study utilized its own methodology, which is associated with its own accuracies and precisions. The specific anatomic parameters addressed in this report include:

Humeral neck angle – defined as the angle between the long axis of the intramedullary canal and the long axis of the humeral head as measured from an anterior-posterior projection;

Humeral head secondary retroversion – defined the angular error of the head osteotomy in the sagittal plane and is formed by the intersection of the resected bone surface (along the anatomic neck) and the face of the humeral stem. Said another way, it provides correction in the incidence of a "go-down" humeral head osteotomy (resulting in a negative angle) or the incidence of a "go-up" humeral head osteotomy (resulting in a positive angle);

Humeral head medial offset – defined as the distance between the long axis of the intramedullary canal and the humeral head center of rotation as measured from an anterior-posterior projection;

Humeral head posterior offset – defined as the distance between the long axis of the intramedullary canal and the humeral head center of rotation as measured from a medial-lateral projection;

Humeral head diameter – defined as the diameter of the head at the level of the anatomic neck;

Glenoid height – defined as the distance from the most superior point on the glenoid to the most inferior point;

Glenoid width – defined as the distance from the most anterior point on the glenoid to the most posterior point.

Glenoid height/width ratio – defined as the ratio of the glenoid height to the glenoid width.

Methodology

This study quantified the proximal anatomy of 49 dried cadaveric humeri and 24 dried cadaveric scapula. These cadaveric specimens were obtained from the anatomy laboratory at the School of Medicine in Bordeaux, France; no demographic data was available for each specimen. The study quantified each of the aforementioned anatomic parameters in each humeral specimen using a Coordinate Measuring Machine (CMM) and its associated software (UMESS Unix Version, Zeiss, Esslingern, Germany); glenoid measurements were obtained with calipers. Measurement reproducibility was determined to be ± 0.75 mm for linear parameters and ± 1.0° for angular parameters.

Results

The results of this study are separated in two sections: the first section discusses the anatomic study of the proximal humerus; the second section discusses the anatomic study of the glenoid.

Proximal Humerus Anatomic Study

The average values, standard deviations, & ranges obtained in the anatomic study of the proximal humerus are described in the following table. Standardized skewness and kurtosis values demonstrated that each parameter, except humeral neck angle & humeral head posterior offset are normally distributed and therefore representative.

Glenoid Anatomic Study

It was observed that nearly every glenoid used in this study was pear-shaped; however, this shape was not always symmetrical relative to the superior/inferior axis. The maximum width is always lower on the anterior edge than the posterior edge of the glenoid. It was observed this difference was relatively constant, having a mean value of 2.6 ± 1.2 mm. The average values, standard deviations, & ranges obtained in the anatomic study of the glenoid are described in the following table. Standardized skewness and kurtosis values demonstrated that glenoid width and the glenoid height/width ratio are normally distributed and therefore representative.

Discussion

The results of this anatomic study of the proximal humerus and glenoid closely match (and are therefore validated by) those of other anatomic studies presented in the literature. The table below compares the results from this study with the weighted averages of the studies described in the background section of this report.

Concerning the anatomic validation of the Equinoxe shoulder prosthesis, a comparison between the geometric variability of the Equinoxe primary prosthesis and that of the anatomic results presented in this study yield the following conclusions (when each parameter is viewed independently):

• The Equinoxe can restore humeral neck angles from 125° to 140°, this variability can successfully reproduce 46 of 49 (94%) humeri used in this study.
• The Equinoxe can restore natural humeral head (secondary) retroversion from -7.5° to 7.5° (assuming the surgeon makes a 0° osteotomy along the anatomic neck) this variability can successfully reproduce 45 of 49 (92%) humeri tested in this study.
• The Equinoxe can restore humeral head medial offset values from -0.5mm - 14.5 mm, this variability can successfully reproduce 49 of 49 (100%) humeri tested in this study.
• The Equinoxe can restore humeral head posterior offset values from -5.95mm to 5.95mm, this variability can successfully reproduce 49 of 49 (100%) humeri tested in this study.
• The Equinoxe can restore humeral head diameter values from 38 mm to 53 mm, this variability can successfully reproduce 47 of 49 (96%) humeri tested in this study.
  

Concerning the glenoid anatomic study, the average glenoid height, glenoid width, and glenoid height to width radio closely match those of other studies presented in the literature. However, it should be noted that the glenoid height & width values of the Equinoxe glenoids (small: 30 mm, 22.2 mm; medium: 34.5 mm, 25.6 mm; large: 39 mm, 28.9 mm) are deliberately shifted to the smaller sizes of the values described in the present study & in each of the aforementioned anatomic studies. This shift is a result of reaming, a standard technique for glenoid resurfacing. Reaming the articular surface often decreases the size of glenoid fossa - the implants are sized according to this fact, downsized by approximately 15%. However, the height/width ratios are irrespective of glenoid reaming; therefore, the values utilized by these implants (1.35) reflect those presented in the present study and those of each aforementioned anatomic study.

Conclusion

Thus it has been quantifiably demonstrated that the Equinoxe total shoulder system provides the capability to successfully restore glenohumeral anatomy in this (and other) representative populations.

References

1. Boileau P, et al. 3-D Geometry of the Proximal Humerus. JBJS. Vol. 79-B, #5: 857-865. 1997.

2. Hertel R, et al. Geometry of the Proximal Humerus and Implications for Prosthetic Design. JSES. Vol. 11, #4: 331-338. 2002.

3. Iannotti JP, et al. The Normal Glenohumeral Relationships. JBJS. Vol. 74-A, #4: 491-500. 1992.

4. Pearl M, et al. Coronal Plane Geometry of the Proximal Humerus Relevant to Prosthetic Arthroplasty. JSES. Vol. 5, #4: 320-326. 1996.

5. Robertson DD, et al. 3-D Analysis of the Proximal Part of the Humerus: Relevance to Arthroplasty. JBJS. Vol. 82-A, #11: 1594-1602. 2000.

6. Roberts SJ, et al. Geometry of the Humeral Head and the Design of Prostheses. JBJS. Vol. 73-B, #4: 647-650. 1991.

7. Sharkey NA, et al. The Rotator Cuff Opposes Superior Translation of the Humeral Head. American Journal of Sports Medicine. Vol. 23, #3: 270-275. 1995.

8. Checroun AJ, et al. Fit of Current Glenoid Component Designs: an Anatomic Cadaver Study. JSES. Vol. 11, #6: 614-617. 2002.

9. Churchill RS, et al. Glenoid Size, Inclination, and Version: An anatomic study. JSES. Vol. 10, #4: 327-332. 2001.

10. Kwon YW, et al. Use of 3-D CT for Analysis of Glenoid Anatomy. JSES. Vol. 14, #1: 85-90. 2004.

11. Mallon WJ, et al. Radiographic and Geometric Anatomy of the Scapula. CORR. #277: 142-154. April 1992.