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48th Annual Meeting of the Orthopaedic Research SocietyPaper No: 0129
ARE BIG HEADS THE SOLUTION TO DISLOCATION AFTER TOTAL HIP REPLACEMENT?+*Noble, P C; *Paravic, V; *Ismaily, S
+*Baylor College of Medicine, Houston, Texas. 713-986-5463, Fax: 713-986-7571, [email protected]
Introduction: With increasing clinical use of new materials in THR,including cross-linked polyethylene and metal-on-metal bearings, surgeonsare looking to large femoral heads to increase range of motion, and decreasethe incidence of dislocation. However, recent studies suggest that the benefitsof increased head size may be limited due to the occurrence of bonyimpingement with increased joint motion. This study was performed using aunique experimental simulation of a sit-to-stand maneuver to assess theimpact of large femoral heads on impingement and dislocation followingTHR.
Materials and Methods: Six fresh cadaveric specimens (3 male, 3 female,average age: 61yrs) were dissected and implanted with a cementless femoralprosthesis with a 38mm head and an acetabular cup oriented in 20° ofanteversion and 45° of abduction. Each pelvis was mounted in a test rig tosimulate motion of the hip joint during rising from a chair. The pelvis wasmounted on the vertical actuator of a bi-axial servohydraulic-testing machine(Bionix, MTS) while the femur was mounted on a linear sliding table. Theprosthetic joint was then reduced and loaded via a system of seven cablesacting in the line of action of the ten major muscle groups active during risingfrom a chair. The forces applied to each cable were calculated fromquantitative EMG measurements performed during sit-to-stand activities(Figure 1).
Figure 1. Test setup used to simulate dislocation.
Under computer control, the hip joint was moved through 60-140 degrees offlexion at 0.5 degrees/second with the femur maintained at 10 degrees ofinternal rotation and six different levels of adduction (15,20,25,30,35, and 40degrees). The occurrence of impingement and dislocation were recordedelectronically and synchronized with measurements of three-dimensionalposition of the femur. The entire test procedure was repeated with a 28mmhead.
Results: There was a dramatic increase in the range of motion of the hip withchange of head size. Over all the positions of hip adduction, hip flexion atimpingement (bony or prosthetic) increased from only 84.6±2.0° with the28mm head to 96.3°±1.9° with the 38mm head, an increase of 11.7°(p<0.001). Hip flexion at dislocation also increased significantly from anaverage of 105.2°±4.0°with the 28mm head to 112.0°±4.0° with the 38mmhead, an increase of 6.8°(p<0.001). The impact of head size varied with the
position of the hip and was greatest at 35 degrees of adduction (28mm: 95.6°±4.7°, 38mm: 106.9°±7.5°, p=0.057).
65
75
85
95
105
115
125
10 15 20 25 30 35 40 45Adduction [deg]
Flex
ion
[deg
]
38 mm 28 mm
Dislocation
Impingement
Figure 2. Hip flexion at impingement and dislocation.
Head size also affected the mechanism of impingement and dislocation of theartificial joint. With the larger head, bony impingement (lesser trochanter/pubis) occurred more frequently leading to rapid loss of joint stability.Prosthetic impingement (neck/ liner) was observed in 94% of the runsperformed with the 28mm heads, but only 64% of runs performed with 38mmheads (Figure 3).
Figure 3. Impingement type.
Discussion: In this study, large heads (38mm) provided increased range ofmotion of the hip to impingement and dislocation. In metal-on-metal andceramic-on-ceramic bearings, increasing head size should reduce theincidence of prosthetic impingement and generation of third body debris. Ourresults also suggest that clinical use of 38mm heads may significantly reducethe incidence of hip dislocation following total hip replacement.**Institute of Orthopedic Research and Education, Houston, Texas.
