Saturday, July 23, 2022

Humeral implants with variable neck shaft angles - is this innovation of value?

The angle between the humeral neck and shaft varies among shoulders.

Implant manufacturers are marketing humeral prostheses with variable neck shaft angles with the goal of matching the native humeral anatomy.

A simpler and less costly approach is to use a prosthesis with a fixed 135 degree neck shaft angle

inserted after making a humeral neck cut at a 45 degree angle with the medullary canal as shown in these drawings by Steven Lippitt from The Shoulder.

The same cut is made whether the native head is in some valgus

or some varus

with the same anatomical result.

The authors of Fixed- vs. variable-angle humeral neck cut in anatomic total shoulder arthroplasty tested this concept in a trial of 50 patients with osteoarthritis having a primary anatomic TSA randomized preoperatively to receive either a variable- or fixed- neck shaft angle (NSA) prosthesis. Humeral neck cut in the variable-NSA group matched the patient’s anatomic neck, with prostheses having a NSA of 127.5, 132.5, or 137.5 degrees. Fixed-NSA cuts were made at 132.5 degrees using an intramedullary guide.

The average preoperative neck shaft angle for all patients was 134 ± 7 degrees.

When comparing average difference in preoperative and postoperative measurements in the fixed-NSA group, the humeral head offset from the humeral shaft axis decreased by 1.4 mm, and the COR moved superiorly by 3.0 mm without significant medialization or lateralization.

The results were essentially the same for the variable angle group: the humeral head offset decreased by 1.2 mm, and the COR moved superiorly by 2.9 mm without significant medialization or lateralization.

The authors concluded that both fixed- and variable-NSA anatomic TSA humeral components demonstrate adequate restoration of premorbid anatomy radiographically.

You can support cutting edge shoulder research that is leading to better care for patients with shoulder problems, click on this link.

Here are some videos that are of shoulder interest
Shoulder arthritis - what you need to know (see this link).
How to x-ray the shoulder (see this link).
The ream and run procedure (see this link).
The total shoulder arthroplasty (see this link).
The cuff tear arthropathy arthroplasty (see this link).
The reverse total shoulder arthroplasty (see this link).
The smooth and move procedure for irreparable rotator cuff tears (see this link).
Shoulder rehabilitation exercises (see this link).

Saturday, March 19, 2016

How important is the humeral head-shaft (neck-shaft) angle in anatomic shoulder arthroplasty?

The rotation of the humeral head does not alter radiographic evaluation of the head-shaft angle.

These authors measured 18 cadaveric humeri. Radiographs were taken with the specimens initially placed in a true anteroposterior position and then subsequently positioned with internal and external rotations of 10°, 20°, and 30°.

For the humerus in the neutral position, the head-shaft angle was 137° ± 4°. With the anatomic specimen positioned with increasing external and internal rotations, there was a maximum difference of only 2° compared with the value observed in the neutral position. The authors concluded that th head-shaft angle did not change significantly with varying degrees of humeral rotation.

Comment: We note that there is substantial variability in the anatomy of the arthritic humerus; the 'anatomic' location of the previously normal articular surface is difficult to determine.

Some surgeons have attached major importance to the humeral head-shaft (neck-shaft) angle in anatomic shoulder arthroplasty. A number of implant systems allow variability in the inclination of the head; however, the effect on the center of rotation of the articular surface seems small.

We use a simpler approach of placing the humeral head at an angle of 45 degrees with the medullary axis of the shaft in all cases, irrespective of the appearance of the preoperative pathoanatomy.

The humeral neck cut is always made at 45 degrees with the orthopedic axis, regardless of whether the preoperative films suggest that the neck-shaft angle is

"normal"