David S. Backman

Internal plate fixation of fractures: short history and recent developments

Metal plates for internal fixation of fractures have been used for more than 100 years. Although initial shortcomings such as corrosion and insufficient strength have been overcome, more recent designs have not solved all problems. Further research is needed to develop a plate that accelerates fracture healing while not interfering with bone physiology.The introduction of rigid plates had by far the greatest impact on plate fixation of fractures. However, it led to cortical porosis, delayed bridging, and refractures after plate removal. These unwarranted effects were said to be caused by bone–plate contact interfering with cortical perfusion. Consequently, further plate modifications aimed to reduce this contact area to minimize necrosis and subsequent porosis.The advocates of limited-contact plates have not published measurements of the contact area or proof of the temporary nature of the porosis. Moreover, clinical studies of newer plate types have failed to show a superior outcome. Histomor-phometric measurements of the cortex showed no difference in the extent of necrosis under plates having different contact areas. Necrosis was predominant in the periosteal cortical half, whereas porosis occurred mostly in the endosteal cortical half. No positive correlation was found between either.The scientific evidence to date strongly suggests that bone loss is caused by stress shielding and not interference with cortical perfusion secondary to bone–plate contact. Consequently, an axially compressible plate (ACP) incorporating polylactide (PLA) inserts press-fit around screw holes was designed. The bioresorbable inserts should allow for (1) increased micromotion in the axial plane to promote healing during the union phase and (2) gradual degradation over time to decrease stress shielding during the remodeling phase.Results of ongoing experimental results are encouraging. Only plates allowing dynamic compression in the axial plane can lead to a revolution in fracture fixation.

Structural disorders at the insertion of the supraspinatus tendon: Relation to tensile strength

We examined macroscopically and microscopically 55 cadaver rotator-cuff tendons attached to their humeral heads to determine the distance between the edge of the articular cartilage and the tendon insertion of the supraspinatus (the width of the sulcus) and the score of regressive changes at the sulcus. In 33 specimens we measured the tensile strength. The width of the sulcus was correlated with the score of regressive changes and with the ultimate tensile strength of the supraspinatus tendon. The width of the sulcus correlated positively with the score of regressive changes (r = 0.66, p < 0.0001), but there was a negative correlation between the latter and the ultimate tensile strength (r = -0.81, p = 0.001) and between the width of the sulcus and the ultimate tensile strength (r = -0.74, p = 0.004). We believe that the width of the sulcus is a simple and useful clinical indicator of the integrity and the tensile strength of the supraspinatus tendon.

Correlation of radiographic measurements with biomechanical test results.

The refracture of bone after the removal of an internal fixation device is not common, but it can be a serious complication. The development of a simple radiographic tool to determine the condition of a healing bone would be of clinical importance. As a basis to developing this tool, it was important to determine whether a correlation exists between radiographic results and the stiffness index of bones during fracture healing. This experimental investigation in beagles showed that at 24 weeks after a unilateral femoral osteotomy, plain radiographs could be used to determine the stiffness index of healing bone. The results showed the cortex to callus ratio (the thickness of the cortex, including the periosteal callus, normalized by the thickness of the cortex of the bone not surgically treated) correlated positively with the stiffness index of the bones. This measurement might be useful in the clinical assessment of fracture healing. From a clinical point of view, caution is recommended after the removal of fracture plates when the cortex to callus ratio is low because such a low ratio may be associated with a low stiffness index of the healing bone. This information allows clinicians to adapt the postoperative regimen accordingly.

Dual cryogenic fixation for mechanical testing of soft musculoskeletal tissues

Mechanical testing of soft musculoskeletal structures like tendons and ligaments are essential to medical advances. A long-standing limitation for testing these structures in isolation has been the ability to solidly fix both ends of the tendon. Cryogenic fixation technology was leveraged into the development of a dual cryogenic fixation (DCF) device. Results of the study show that the DCF allows tendons to be tested in isolation, at physiologic temperatures, with excellent reproducibility.