Three-dimensional mechanical evaluation of joint contact pressure in 12 periacetabular osteotomy patients with 10-year follow-up

Figures & data

Figure 1. Angles describing the orientation of the acetabulum and femoral coverage are calculated from a 3D segmented model of the pelvis in frontal, sagittal, and transverse planes emulating those traditionally measured from radiographs or reformatted CT scans. In the frontal plane (top left), the AC angle measures the obliqueness of the acetabular roof between the medial aspect of the sourcil, the lateral edge of the acetabular rim, and a horizontal line. The CE angle measures the coverage of the lateral edge of the acetabulum with respect to the center of the femoral head (not shown), and may have a negative value in severely dysplastic cases. The orientation of the acetabular cup (S-AC angle) in the sagittal plane (top right) is measured from the most superior aspect of the acetabular roof to the most anterior aspect with respect to horizontal. The anteversion (AcetAV angle) is measured on a transverse plane (bottom left) viewed inferior to superior using a line created between the posterior and anterior rim of the acetabulum. The angle is measured with respect to a line normal to the two femoral head centers (intercapital centerline). The angle measurements help to characterize the orientation of the load-bearing surface of the hip joint in 3 dimensions (bottom right) during realignment (axis scale in mm; viewed isometric from anterio-lateral-superior).

Table 1. Clinical results for the 12 patients

Patient no.	1	2	3	4	5	6	7	8	9	10	11	12
q-score
preoperatively	76	75	62	75	–	–	68	–	74	–	–	51
2-year	99	99	68	100	45	90	86	100	–	86	–	99
10-year	100	86	100	97	–	100	100	100	70	83	98	100
Harris:
2-year	100	100	100	100	52	96	96	100	100	97	100	100

Table 2. Radiological results for the 12 patients. The radiological angles associated with planning of the PAO are listed, including the center–edge (CE), acetabular inclination (AC), superior–anterior angle (S–AC) and acetabular anteversion (AcetAV)

Patient no.	1	2	3	4	5	6	7	8	9	10	11	12
CE angle (°)
preoperatively	14	10	14	6	21	6	2	–6	19	14	15	3
postoperatively	24	30	25	23	47	27	23	35	42	36	35	24
Optimum	26	27	26	27	38	25	24	22	26	25	30	27
AC angle (°)
preoperatively	24	24	21	32	21	26	21	42	8	16	22	34
postoperatively	13	3	11	6	–13	–2	4	5	–11	1	–1	7
Optimum	8	5	8	12	3	2	1	3	3	5	4	2
S-AC angle (°)
preoperatively	–26	–25	–35	–34	–33	–9	–6	N ^a	–28	–23	–26	–1
postoperatively	–30	–32	–33	–43	–37	–38	–38	–26	–31	–31	–30	–24
Optimum	–34	–35	–37	–34	–40	–31	–32	–23	–34	–26	–35	–31
AcetAV (°)
preoperatively	28	19	27	20	20	26	18	29	16	17	29	38
postoperatively	27	11	30	N	N	1	6	N	25	15	N	N
Optimum	16	14	30	N	N	19	17	N	11	18	25	N

^a N = Not a number; angle undefined.

Figure 2. Mechanical results in which the postoperative outcome matched closely with the optimum contact pressure profile (patient 11). The blue line shown passing through the joint contact surface represents the force applied through the hip, while the green line represents the displacement direction of the femoral head. In the optimized case, the lines are collinear. Scale units are MPa.

Table 3. Biomechanical and radiological results for the 12 patients. The peak pressure and contact area of the acetabulum are listed. Additional parameters include the centroid of pressure (CP) and the force-displacement separation angle (RU)

Patient no.	1	2	3	4	5	6	7	8	9	10	11	12
Pressure (MPa)
Preoperatively	2.9	2.8	4.1	2.6	2.1	5.0	5.1	7.7	2.0	2.3	2.0	4.5
postoperatively	2.1	1.6	3.1	1.5	1.8	2.1	2.1	3.2	2.1	1.9	1.5	1.5
optimum	1.9	1.6	3.1	1.4	1.3	2.0	2.0	1.6	1.8	1.7	1.3	1.4
Area (mm^2)
preoperatively	1562	1678	1482	1937	2153	1175	1234	535	1920	1836	1984	1207
postoperatively	1824	2367	1677	2826	2861	2260	2395	1496	2370	2647	2631	2689
optimum	2144	2390	1821	2739	2904	2242	2395	1744	2255	2351	2741	2845
CP ratio
preoperatively	0.12	0.03	0.02	0.02	0.03	0.05	0.01	0.12	0.26	0.02	0.03	0.09
postoperatively	0.65	0.74	0.55	0.56	1.02	0.79	0.68	1.12	1.12	0.84	0.92	0.58
optimum	0.63	0.69	0.70	0.60	0.64	0.82	0.83	0.70	0.71	0.70	0.68	0.71
RU angle (°)
preoperatively	36	45	33	45	37	59	60	72	20	35	39	61
postoperatively	13	5	4	7	34	7	5	50	24	13	24	13
optimum	0	0	0	1	0	1	0	0	0	1	1	0

Figure 3. An example PAO case in which the postoperative pressure profile indicates overcorrection of the bone fragment (patient 5). The preoperative case shows pressure concentration on the lateral aspect of the joint contact surface. Excessive adduction of the acetabular fragment resulted in medially concentrated pressure postoperatively. Units are MPa.