Implant position calculation for acetabular cup placement considering pelvic lateral tilt and inclination

Figures & data

Figure 1. Sawbone pelvis model: Attached plumb line, pelvis frontal plane, and acetabular cup with rod indicating its inclination and anteversion angles.

Figure 2. Vertical plane of the sawbone model.

Figure 3. Visualized frontal plane of the pelvis.

Figure 4. Increasing pelvic inclination leading to a change in anteversion.

Figure 5. Microsoft Excel program screenshot showing the results of calculating the projected cup positions in a pelvis with 20° of inclination and left hip elevated 15°.

Figure 6. Acetabular cup inclination in relation to pelvic inclination. Values are given in degrees.

Table I.  Projected implant positions in the vertical plane with acetabular cup position of 45° inclination / 15° anteversion with regard to the pelvis frontal plane and different grades of pelvic inclination.

Pelvic inclination (°)	Projected cup inclination (°)	Projected cup anteversion (°)
− 20	37.0	31.2
− 15	39.5	27.8
− 10	41.7	23.9
− 5	43.5	19.7
0	45.0	15.0
5	46.1	10.0
10	46.7	4.8
15	46.9	− 0.5
20	46.6	− 5.8
25	45.9	− 11.0
30	44.7	− 15.9
35	43.2	− 20.5
40	41.2	− 24.7
45	39.0	− 28.5

Figure 7. Acetabular cup anteversion in relation to pelvic inclination. Values are given in degrees.

Table II.  Calculated cup positions in relation to the pelvis frontal plane for a projection of 45° inclination / 15° anteversion in the vertical frontal plane for different degrees of pelvic inclination.

Pelvic inclination (°)	Required cup inclination (°)	Required cup anteversion (°)
− 20	46.6	− 5.8
− 15	46.9	− 0.5
− 10	46.7	4.8
− 5	46.1	10.0
0	45.0	15.0
5	43.5	19.7
10	41.7	23.9
15	39.5	27.8
20	37.0	31.2
25	34.3	34.2
30	31.4	36.9
35	28.3	39.1
40	25.1	41.0
45	21.8	42.7

Table III.  Influence of leg length difference on pelvic lateral tilt angle.

Leg length difference (cm)	Pelvic lateral tilt angle (°)
0	0
1	3.82
2	7.66
3	11.54
4	15.47
5	19.47

Table IV.  Effect of pelvic lateral tilt and inclination on projected implant positions in the vertical plane with bilateral acetabular cup positions of 45° inclination / 15° anteversion with regard to the pelvis frontal plane.

Pelvic inclination (°)	Pelvic lateral tilt (°)	Right hip cup inclination (°)	Right hip cup anteversion (°)	Left hip cup inclination (°)	Left hip cup anteversion (°)
0	5	40.2	16.5	49.8	13.9
0	10	35.4	18.4	54.7	13.0
0	15	30.7	21.0	59.6	12.3
0	20	26.1	24.6	64.5	11.7
5	5	39.0	11.1	48.8	9.2
5	10	34.1	12.4	53.8	8.6
5	15	29.2	14.3	58.7	8.1
5	20	24.4	17.0	63.7	7.7
10	5	38.3	5.3	48.3	4.4
10	10	33.3	6.0	53.2	4.1
10	15	28.3	6.9	58.2	3.9
10	20	23.4	8.3	63.2	3.7
15	5	38.1	− 0.6	48.1	− 0.5
15	10	33.1	− 0.7	53.1	− 0.4
15	15	28.1	− 0.8	58.1	− 0.4
15	20	23.1	− 0.9	63.1	− 0.4
20	5	38.4	− 6.5	48.3	− 5.4
20	10	33.4	− 7.3	53.3	− 5.0
20	15	28.5	− 8.4	58.3	− 4.7
20	20	23.5	− 10.1	63.3	− 4.5