Influence of hip center position, anterior inferior iliac spine morphology, and ball head diameter on range of motion in total hip arthroplasty.

BACKGROUND

Acetabular component orientation, such as high placement and femoral head diameter influence joint stability in total hip arthroplasty (THA), wherein anterior inferior iliac spine (AIIS) shape could cause femoro-acetabular impingement. Little is known regarding the combined influence of these parameters, particularly in the context of developmental dysplasia of the hip. Therefore we conducted a computer simulation study based on computed tomography (CT) data to determine whether: (1) AIIS shape, (2) high placement of acetabular cups, and (3) ball head diameter influence the range of motion (ROM) after THA.

HYPOTHESIS

The decrease in ROM depends on AIIS shape and the ROM decreases even if the femoral head diameter is increased when high placement of acetabular cups.

PATIENTS AND METHODS

CT data from 14 hips of 14 patients were evaluated. Hips were categorized by Hetsroni classification type I (n=6), type II (n=6), and type III (n=2) depending on AIIS shape. ROM was evaluated using CT-based software. Cups were placed at and 5 and 10mm above the normal hip position. The femoral heads used were 28 (standard simulation), 32, and 36mm in diameter. ROM at impingement was measured under flexion (Flex), internal rotation (IR) at 90° flexion (IR at 90Flex), IR at 45° flexion with a 20° adduction (IR at 45Flex20Add), and external rotation at 10° extension (ER at 10Ext).

RESULTS

The mean ROM standard simulation for Flex, IR at 90Flex, IR at 45Flex20Add and ER at 10Ext were: 119.8±5.4°, 31.0±11.3°, 70.0±11.9°, and 33.0±9.7° for type I; 118.5±5.5°, 31.5±2.9°, 71.3±2.2°, and 33.3±3.3° for type II; and 105.5±13.4°, 21.0±15.6°, 61.0±11.3°, and 34.5±2.1° for type III, respectively. There were no significant differences in the ROMs of each type (Flex, p=0.252; IR at 90Flex, p=0.461; IR at 45Flex20Add, p=0.261; and ER at 10Ext, p=0.655). For the high placement of acetabular cups, ROM increase was restricted despite the femoral head diameter increase.

DISCUSSION

Larger femoral head diameters increased ROM, with a lower increase in type III because of bony impingement. ROM decreased with higher cup placement. Expansion effects were minimal, even with larger ball head diameters, and were further decreased in types II and III. Attention should be paid to AIIS shape because bony impingements occur early with higher acetabular cup placement.

LEVEL OF EVIDENCE

VI Simulation study.