Schulterendoprothetik

Biomechanical testing of rectangular humeral shaft prosthesis: higher torsional stability without increased fracture risk

BACKGROUND

Rectangular cementless femur shaft prostheses have a higher primary stability than round shafts. A novel rectangular humeral shaft design was tested with two questions: does the rectangular design cause a higher fracture risk during implantation than round designs, and does it increase the torsional stiffness?

MATERIALS AND METHODS

Two series with six paired human humeri (total 24) were tested on one side with the rectangular shaft and on the contralateral side with a round shaft. In the first series, the shaft implantation was carried out with a constant speed of 100 mm/min and the maximum force was measured when the fracture occurred. In the second series, the implants were preloaded with 50 N and then rotated at 2° per second with monitoring of the torsional torque.

RESULTS

The maximum force at fracture showed no significant difference for the two designs (p = 0.34). Higher age and low bone density reduced the force required for fracture. The rectangular shaft showed significant higher torsional moments (p < 0.05).

CONCLUSIONS

In biomechanical testing, the rectangular shaft had a significantly higher primary torsional stability than the round shaft without a higher risk of fracture during cementless implantation. Fracture risk and torsional stability are influenced by age and bone density.

Use of the fulcrum axis improves the accuracy of true anteroposterior radiographs of the shoulder

In 100 patients the fulcrum axis which is the line connecting the anterior tip of the coracoid and the posterolateral angle of the acromion, was used to position true anteroposterior radiographs of the shoulder. This method was then compared with the conventional radiological technique in a further 100 patients.

Three orthopaedic surgeons counted the number of images without overlap between the humeral head and glenoid and calculated the amount of the glenoid surface visible in each radiograph. The analysis was repeated for intraobserver reliability. The learning curves of both techniques were studied.

The amount of free visible glenoid space was significantly higher using the fulcrum-axis method (64 vs 31) and the comparable glenoid size increased significantly (8.56 vs 6.47). Thus the accuracy of the anteroposterior radiographs of the shoulder is impaired by using this technique. The intra and interobserver reliability showed a high consistency. No learning curve was observed for either technique.

[Shoulder replacement in arthritis of the shoulder joint]

Primary arthritis of the shoulder is one of the main indications for implantation of a shoulder endoprosthesis. The shoulder endoprostheses currently preferred have developed through four generations from the original monobloc prostheses to modular prostheses. Fourth-generation models allow the implant to be adjusted to suit the anatomical situation of the shoulder joint, so that physiological tensions and the centre of rotation can be restored. Conventional X-ray exposures in a-p and axial projections and an MRI investigation are considered standard parts of the surgical planning, and a CT examination is also performed when bony defects are present. The deltoid-pectoral approach is recommended for implantation of the prosthesis. In most cases the cup has also undergone arthritic alterations, which means that a total prosthesis is indicated. Results with total prostheses are superior to those obtained with hemi-endoprostheses. In the older patient the stem of the prosthesis is mostly cemented in, while cement-free fixation is also possible in the younger patient with good bone quality. The rates of loosening found in some 10-year follow-up studies of patients with cemented prostheses are well under 1%. The weak point of a shoulder endoprosthesis is still the replacement of the glenoid. The implantation is technically challenging. Cemented glenoid replacement is regarded as standard. Clinically significant loosening has been found to occur in up to 10% of cases in 10-year follow-up studies, and in up to 80% of cases in which there are radiologically demonstrable lytic borders. The newest development is that of humeral head implants that are anchored in the metaphysis without cement. These implants can be expected to be implanted in increasing numbers of patients with good bone quality in the future.

[Hemiarthroplasty or total shoulder replacement in glenohumeral osteoarthritis?]

Shoulder arthroplasty has proven to be a successful option in the treatment of glenohumeral osteoarthritis. Although introduced in the 1970s by Ch. Neer, resurfacing of the glenoid remains controversial. With regard to some single prospective studies total shoulder replacement (TSR) seems to be superior to humeral head replacement (HHR) in the treatment of concentric osteoarthritis. The current literature shows that TSR leads to a better shoulder function and less pain. The decision on whether to perform a TSR or HHR should be guided by the individual pathology. The final treatment should be reliable in terms of a better shoulder function, reduced pain, and good strength. The role of glenoid loosening in TSR and its impact on shoulder function in the long term remains uncertain and has to be clarified in the future.