Evaluation of the appropriate reaming diameter during initial fixation of a cementless hip prosthesis
Background: Press-fit fixation is important technical factor to achieve initial stability of a cementless acetabular cup for good clinical results of total hip arthroplasty. However, appropriate reaming diameter during initial fixation is unclear. Therefore, this study aimed to evaluate the optimal reaming diameter using simulated bones and cementless cups. Methods: Three types of simulated bones with different degrees of hardness were used (10 pcf, 20 pcf, 30 pcf, pcf = 16.02 kg/㎥). Acetabular models were created by reaming the simulated bone into a hemisphere, and the reaming diameters were 48 mm, 49 mm, and 50 mm.The 50 mm diameter acetabular cup was fixed to simulated bones with a compressive load of 16,000 N at a rate of 12 mm/min. The testing machine was attached to a cup fixed to the simulated bone, and a pull-out test, rotation test, and lever-out test were performed. To evaluate the initial gap, ink was applied to the cup surface during the pull-out test, and the contact between the bone and cup was visually evaluated after pull-out. Results: The pull-out load of the 20 and 30 pcf simulated bones was significantly lower at a reaming diameter of 50 mm that those at reaming diameters of both 48 and 49 mm (P < 0.05). The rotational torque of the 20 and 30 pcf simulated bones was significantly lower at a reaming diameter of 50 mm that those at reaming diameters of both 48 mm and 49 mm (P < 0.05). The lever-out moment of the 20 and 30 pcf simulated bones was significantly lower at a reaming diameter of 50 mm than those at reaming diameters of both 48 mm and 49 mm (P < 0.05). Contact between the 30 pcf simulated bone and the cup at a reaming diameter of 48 mm was mainly at the edge of the cup; contact at the center of the cup was poor. Conclusions: We performed mechanical tests using simulated bones and evaluated the initial fixation of the cup according to the bone reaming diameter. We recommend under-reaming by 1 mm in all cases to optimize both initial fixation capacity and contact between acetabular cup and bone.