Healio
Healio
Issue: December 2004
ByPascal Boileau, MD
December 01, 2004
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Prosthetic adaptability

Issue: December 2004
ByPascal Boileau, MD
Pascal Boileau, MD Pascal Boileau, MD,
is professor and head of the Department of Orthopedic Surgery and Sports Traumatology, Archet 2 Hospital in Nice, France.

A nonconstrained shoulder prosthesis is the preferred treatment for diseases such as primary osteoarthritis, rheumatoid arthitis and avascular necrosis, in which the glenohumeral joint is destroyed but the rotator cuff muscles and tendons are intact and functional.

My colleagues and I advanced the design of the nonconstrained shoulder prosthesis to restore normal shoulder anatomy and kinematics. The Aequalis shoulder prosthesis (TORNIER, St. Ismier, France) is a nonconstrained glenohumeral prosthesis. Its development is based on one feature necessary to produce consistent clinical results — the development of a humeral implant that matches the anatomy of the proximal humerus, based on the concept of prosthetic adaptability.

First- and second-generation prostheses

The first nonconstrained shoulder prostheses, such as the Neer prosthesis, were one piece (monoblock). With these prostheses, it was difficult to reproduce the anatomy and 3-D geometry of the proximal humerus. The second generation of shoulder prostheses incorporated the concept of modularity: The prosthetic head and stem had different sizes and could be connected together by a Morse taper.1

Modularity of humeral components in shoulder prostheses represented an attempt to match the variable dimensions of the articular surface and intramedullary canal but was not a guarantee of restoring the normal anatomy. As demonstrated by a computerized anatomical study, the shape of the proximal humerus is more variable than most modular and nonmodular components can accommodate.2-4

Third-generation prostheses

My colleagues and I developed and proposed adaptability of the humeral component to restore the variable shape of the proximal humerus. An adaptable humeral implant should permit reproduction of the proximal humeral anatomy. With regard to shoulder prosthetic design, the adaptability of the humeral implant must allow integration of different parameters.

The prosthetic stem must be shortened to allow the prosthesis to better adapt to the anatomy of the proximal humeral diaphysis. The curvature of the humerus is located approximately 125 mm from the epiphysis. When the stem of a prosthesis is introduced beyond that curvature, the implant assumes either a varus or valgus position. The most problematic features of second-generation designs were that they had a long stem and they were press-fitted (i.e., the position of the stem prosthesis dictates the position of the prosthetic head).2-5 If the humeral head center lies too superiorly, then the subscapularis and infraspinatus convert from abductors into adductors and substantially increase the load on the supraspinatus for elevation and abduction.6,7 The 105-mm stem of the Aequalis prosthesis is intentionally short to adapt to the anatomy before the change of curvature at the humeral diaphysis.

Our studies have shown that a linear relationship exists between humeral head thickness and diameter. Only one diameter is possible for one thickness, except for the large head sizes (diameter <50 mm). Second-generation prosthetic designs were based on the concept of knee replacement and usually provided different thicknesses for the same diameter. This led to oversized implant heads that had several consequences1: over-tension of the joint, which limits shoulder mobility and possibly increases glenoid cartilage or polyethylene wear, and overtension of the cuff, which can be responsible for the early rupture of the subscapularis repair and, later, supraspinatus tears. Thus, the selection of the right size prosthetic head is critical. Biomechanical experiments have demonstrated that a change in the position of the center of rotation by 5 mm to 10 mm results in a significant reduction of the lever arm lengths of the deltoid and the rotator cuff muscles during abduction. The Aequalis prosthesis has seven articular heads of increasing size, the diameter and thickness of which were determined from our anatomical studies. A single head thickness is used for each so that the anatomical variations encountered can be better matched.

Figure 1A Figure 1B Figure 1C

Figure 1. Translation of the articular head surface related to the theoretical stem axis shown on radiographs: medial offset (A), posterior offset (B) and combined offset (C).

Figures courtesy of Pascal Boileau, MD.

The center of rotation of the humeral head must be relocated. With regard to shoulder prosthetic design, the adaptability of the humeral implant must allow integration of four geometrical parameters — inclination, retroversion, mediolateral offset and anteroposterior offset of the proximal humeral articular surface.

The inclination of the articular head surface is highly variable, ranging in our study from 114° to 147°.2-4 The fixed inclination with most modular and nonmodular prostheses limits the potential for a surgeon to reproduce the original anatomy. Making a humeral cut of 130° of inclination in a patient who has 140° displaces the center of rotation distally and alters the kinematics of the glenohumeral joint.8-10

The design of the Aequalis prosthesis incorporates the variable inclination by using variably angled neck components between the stem and the prosthetic head. The inclination is chosen preoperatively using a template. In our study, we demonstrated that when four types of humeral stem-neck angles are provided, more than 95% of patients can be covered.2-4 Surgeons should be aware that a prosthesis design that does not respect the variable neck-shaft angle cannot be truly considered a third-generation prosthesis.

Figure 2 Figure 3A Figure 3B

Figure 2. The first adaptable prosthesis with variable inclination and offset.

Figure 3. An example of primary osteoarthritis (A) and anatomical reconstruction with the Aequalis prosthesis (B).

Figures courtesy of Pascal Boileau, MD.

The retroversion of the articular head surface is also highly variable, ranging in our study from -6° to 46°. Imposing a fixed and arbitrary humeral retroversion ranging between 30° and 40° may displace the center of rotation and modify the kinematics of the glenohumeral joint.6-10 Also, alteration of the bony anatomy by changing humeral retroversion may lead to eccentric loading at the periphery of the glenoid, possibly increasing glenoid wear and glenoid loosening.6,7

The retroversion and the inclination of the articular head surface are variable factors that should be individualized for each patient. Marking the anatomical neck is a fundamental stage during shoulder arthroplasty, as it allows the individual humeral inclination and retroversion to be determined so that the orientation in the space of the articular surface may be fixed.

The combined offset of the articular head surface must also be considered in the design of the humeral implant (Figure 1). In certain patients, during implantation of a standard humeral prosthesis, the prosthetic head is pushed forward so that it does not cover the bone cut.11 The solution can involve posterior translation and centralization of the prosthesis artificially increasing retroversion after having modified the bony section to accommodate this new adaptation. Alternatively, a variable offset can be incorporated in the prosthesis design. A variable offset is selected by the surgeon preoperatively, using an original eccentric indexing system, located on the inferior aspect of each head (Figure 2). Eight positions are available allowing the articular head to be offset posteriorly or anteriorly and medially or laterally, until the cut bony surface is perfectly covered (Figure 3). The original prosthetic head eccentric displacement system allows consideration of the variable location of the articular head surface.

Conclusion

The Aequalis prosthesis is a third-generation nonconstrained prosthesis with a design that allows restoration of the 3-D geometry of the proximal humerus through prosthetic modularity and adaptability.

References

  1. Neer CS. Shoulder arthroplasty: History, new designs and new complications. In: Mansat M, ed. Prothèses D'épaule. Paris: Expansion Scientifique; 1999:1-11.
  2. Boileau P, Walch G. Adaptabilité et modularité au cours des prothèses d'épaule. Acta Orthop Belg. 1995;61(Suppl I):49-61.
  3. Boileau P, Walch G. Three dimensional geometry of the proximal humerus: Implications for the surgical technique and prosthetic design. J Bone Joint Surg Br. 1997;79(5):857-865.
  4. Boileau P, et al. Normal and pathologic anatomy of the glenoid: effects on the design, preparation and fixation of the glenoid component. In: Walch G, et al, eds. Shoulder Arthroplasty. New York: Springer-Verlag; 1999:127-140.
  5. Ballmer FT, et al. Humeral head prosthetic arthroplasty: sugically relevant geometric considerations. J Shoulder Elbow Surg. 1993;2:296-304.
  6. Nyffeler RW, et al. Influence of version loading pattern of glenoid component: an experimental investigation. J Shoulder Elbow Surg. 2000;96: 549-550.
  7. Nyffeler RW, et al. Influence of humeral prosthesis height on biomechanics fo glenohumeral abduction. An in vitro study. J Bone Joint Surg Am. 2004;86(3):575-580.
  8. Pearl ML, Volk AG. Retroversion of the proximal humerus in relationship to prosthetic replacement arthroplasty. J Shoulder Elbow Surg. 1995;4:286-289.
  9. Pearl ML, Volk AG. Coronal plane geometry of the proximal humerus relevant to prosthetic arthroplasty. J Shoulder Elbow Surg. 1996;5:320-326.
  10. Pearl ML, Kurutz SA. Geometric analysis of commonly used prosthetic systems for proximal humeral replacement. J Bone Joint Surg Am. 1999;81:660-671.
  11. Roberts SNJ, et al. The geometry of the humeral head and the design of prpostheses. J Bone Joint Surg Br. 1991;73:647-650.