Wednesday, April 6, 2011

How is the roundness of the humeral head restored in shoulder arthritis?

One of the major barriers to regaining motion in the arthritic shoulder is that the ball of the joint (the humeral head) is no longer smooth and round as shown in the surgical photo below.

As shown below, a metal humeral head prosthesis (cobalt chrome alloy) can restore the roundness and smoothness of the humeral side of the joint.

The humeral head prosthesis is connected to a stem (titanium alloy) the tapered body of which fits inside the humerus.

The inside of the humerus (arm bone) may be cylindrical or tapered.
The inside of the humerus is not of a consistent shape.
Trying to fit a prosthesis by reaming the inside of the bone may substantially weaken it.

Trying to force a tight fit risks fracture.

We find that the safest method for securing the stem within the humeral canal is to use impaction grafting with bone harvested from the arthritic humeral head (which is removed at the time of surgery) to conform the inner surface of the bone to the prosthesis. Some have likened this fitting of the patient's bone the prosthesis to the fitting of the traveler to the bed by the inn keeper Procrustes.
As a result, the tapered stem is securely fixed with a biological press fit that safely distributes the load from the prosthesis to the humerus.

We have published research showing that neither cement nor bone ingrowth is required to securely fix the stem in the humerus as long as the stem has a tapered upper end. We have shown that this impaction grafting improves the fit of uncemented humeral arthroplasty. We have also pointed to the importance of avoiding over reaming the bone in an attempt to achieve secure fixation. 

Impaction grafting is the safest way to fix the stem in that it builds up the inside of the bone - making it stronger - while achieving a secure press fit. Finally, should the stem require revision for any reason, it can be removed without damaging the bone - in contrast to splitting the bone (shown below) which may become necessary in removing a cemented or bone-ingrowth stem.


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