Biomedical Engineering Reference
In-Depth Information
7.5 Design Evaluation
The primary lessons of these early designs are:
a) Avoid unnecessary prosthetic constraints
where possible. Allow the soft
tissue constraints to act. If the soft tissue constraints are not available and
prosthetic constraints must be used, provide sufficient fixation to resist the
expected loading resulting from these constraints.
b) Accommodate normal motion and loading.
Failure to provide for normal
motion and loading can produce functional deficiencies, excessive stresses in the
prosthesis and at the bone-prosthesis interface.
c) The design of a successful shoulder replacement is complex
and careful
design is necessary to avoid serious design defects.
7.5.1 Current Generation Designs
It is useful to evaluate the current devices in light of Design Considerations and
Objectives for Glenohumeral Replacement of Section 7.4.5 and the lessons above
in order to attempt to establish the appropriate utilization of these devices.
7.5.1.1 Humeral Resurfacing Cups
As a class, humeral resurfacing cups most closely approach the design ideals of
secion 7.4.5 and incorporates the lessons of the early experiences with shoulder
arthroplasy. They are the simplest to manufacure, implant and use, preseve the
most bone, allow restoration of normal function, simplest to revise with minimal
damage and have excellent fixation stability. They may be used as
hemiarthroplasties or total arthroplasties if the are used with a glenoid component.
As a result they have excellent clinical performance and longevity. [8-11, 25, 26]
The original Buechel-Pappas Humeral Resurfacing Cups are unchanged since
their introduction in the early 1980's. They are made of ceramic coated titanium
alloy and available in four sizes. The interior of the cup is porous coated with
sintered titanium beads for biological fixation. The edge of the cup is beveled and
rounded, as illustrated in Fig. 7.12.
This is done to avoid destructive edge-type impingment with the glenoid
articulating surface or bearing and with any adjacent soft tissue.[27] The first
author has used these devices for a quarter century with excellent results. An
independent study of this device by Bodell and Joseph [11] also described
excellent clinical results.
The simplicity of implantation and preservation of bone is illustrated in
Fig. 7.17.
