OTA 1997 Posters - Trauma Issues

Intramedullary Reamer System Design and Performance: Comparison Study and Clinical Implications

David M. Banks, MD, Matthew E. Harrow, BS, Richard D. Peindl, PhD, Michael J. Bosse, MD, James F. Kellam, MD

Carolinas Medical Center, Charlotte, North Carolina, USA

Purpose: To assess the relative advantages/disadvantages of individual reamer system designs, provide data to permit further optimization of these designs, and detemine potential changes in clinical practice which might minimize embolization of intramedullary contents.

Methods: Four newly acquired, commonly used reamer systems were tested in the size range of 9 mm - 14 mm diameter, in 0.5 mm increments. Representative reamer head geometry for the four systems tested are shown in Fig. 1 as A, B, C, and D. All tests were performed on a custom-instrumented milling machine. Constant reamer feed rates of 1.0 cm per second and 7.6 cm per second, were selected to simulate cautious and vigorous reaming conditions as observed clinically. Tests were performed at cutter speeds of 250 rpms and 750 rpms in order to evaluate reamer mechanical loading at representative reaming and drilling speeds since it was hypothesized that higher cutter speeds might be appropriate for the smaller reamer head sizes. Each reamer set was tested in six predrilled hard oak blocks used to simulate cortical bone. Data were collected documenting axial load, torsional load, reamer speed and reamer feed rates. In total, approximately 1000 tests were performed and analyzed.

Results: In our investigations several general observations can be made with regard to all of the reamer head designs tested. For the four systems tested, the two systems with the deepest flutes generated the lowest mean axial and torsional loads. All systems performed better (i.e. reduced maximal axial loads and torques) at the smaller reamer sizes at drilling speeds rather than reaming speeds. Each system tested demonstrated a variable sensitivity to changes in operating speeds as evidenced by variable increases or decreases in mean and peak-to-peak loads. These results indicate that certain designs are more dynamically stable and less speed dependent. Overall, the deep-fluted design with a long lead taper and round, burr-like body shape consistently produced minimal mechanical loading. This same feature, however, can result in high peak-to-peak (chatter) loads/torques if the flutes are not tapered. Two of the four reamer systems tested (B and C) utilize a larger flex shaft diameter for the larger reamer head sizes (> 13 mm). Combined axial loading and torsional data do not appear to indicate a need for a larger flex shaft for larger reamer head sizes. Maximal loads and torques indicate decreases for axial loads and minor variations in measured torques for the larger reamer sizes at typical reamer speeds.

Discussion: For multiply-injured patients, the current literature supports early internal stabilization of all long bone fractures. Currently it is unclear, however, whether reamed or unreamed intramedullary nailing is preferable. While it is generally accepted that a large diameter, locked intramedullary nail results in greater fracture stabilization as compared to a loose fitting, locked nail, controversy still exists concerning the morbidity or mortality associated with embolization of marrow contents concurrent with intramedullary pressurization. It has been shown that the flexible drive shaft diameter is a major factor affecting intramedullary pressurization and blunt reamers cause increased intramedullary pressure, greater tangential strain on the diaphysis, and larger increases in cortical temperature. In addition, comparison studies have demonstrated significant differences in intramedullary pressure, fat embolism, and pulmonary arterial pressure between different commercially available reamer system designs.

Conclusion: Recently, there have been several reports concerning the assessment and outcomes between reamed and unreamed femoral nailing procedures. Interestingly, there is little mention as to which reamer system designs were utilized for these investigations despite literature demonstrating differences in performance between differing reamer systems. There are a number of variable factors that contribute to the ultimate pressure level in the medullary cavity, some which are operator dependent and others which are a direct consequence of reamer design. Based on our understanding of the biomechanical properties/actions of reamers in conjunction with the currently available clinical data, if asked to select a reamer for clinical use today, we would suggest a reamer system that incorporates a tapered front cutting design with deep cutting flutes. We also would suggest that higher reaming speeds may be beneficial for the small head sizes (9-10 mm), but further animal/clinical trials are required.