From June 2003 to December 2017, 23 patients (27 hips) with PA, underwent primary THA at our institution. A diagnosis of PA was made when there was an intrapelvic displacement of the femoral head medial to the ilioischial line (Kohler’s line) on anteroposterior (AP) radiograph of hip (Fig. 1) [1, 6]. All of them were operated with cementless prostheses.
These patients were followed up for 2 to 16 years (average, 5.2 years) after the index THA. There were 5 men (7 hips) and 18 women (20 hips), their mean age at the time of THA was 60.5 years (range, 24.5 to 77 years), and their mean body mass index was 22.3 kg/m2 (range, 16.0 to 29.7 kg/m2) (Table 1).
Table 1
Demographics of patients with protrusio acetabuli
|
Overall
(n = 27)
|
Medialized group
(n = 9)
|
Restored group
(n = 18)
|
p-value
|
Gender Male
Female
|
7
20
|
2
7
|
5
13
|
0.756
|
Age
|
60.5 ± 15.0
|
64.6 ± 12.7
|
58.5 ± 15.9
|
0.348
|
BMI
|
22.3 ± 3.4
|
23.3 ± 2.6
|
21.8 ± 3.8
|
0.375
|
Diagnosis
|
|
|
|
0.905
|
Rheumatoid arthritis
|
14
|
5
|
9
|
Post-traumatic arthritis
|
7
|
4
|
3
|
Ankylosing spondylitis
|
4
|
0
|
4
|
Post-radiation osteonecrosis
|
1
|
0
|
1
|
Previous infection
|
1
|
0
|
1
|
Grade
|
|
|
|
0.107
|
Mild (< 5mm)
|
5
|
0
|
5
|
Moderate (6-15mm)
|
20
|
8
|
12
|
Severe (> 15mm)
|
2
|
1
|
1
|
Cup abduction (°)
|
40.8 ± 6.9
|
37.4 ± 5.2
|
42.4 ± 7.1
|
0.023
|
Cup anteversion (°)
|
24.4 ± 9.2
|
23.7 ± 8.5
|
24.7 ± 9.7
|
0.820
|
Postoperative LLD (cm)
|
0.6 ± 1.0
|
0.2 ± 0.9
|
0.8 ± 1.0
|
0.131
|
Follow-up duration (years)
|
5.2 ± 3.3
|
5.3 ± 3.5
|
5.1 ± 3.1
|
0.875
|
mHHS
|
82.4 ± 11.6
|
83.6 ± 12.1
|
81.8 ± 10.8
|
0.498
|
Abbreviations: BMI body mass index; LLD leg-length discrepancy, mHHS modified Harris hip score
|
The causes of PA were rheumatoid arthritis in 14 hips, post-traumatic arthritis in 7 hips, ankylosing spondylitis in 4 hips, post-radiation osteonecrosis in 1 hip and previous infection in 1 hip.
We measured the amount of acetabular protrusion on hip AP view. Theoretically, the inner wall of the acetabulum, which appears as the pelvic tear-drop on the AP radiograph, would be the ideal reference structure to measure the amount of acetabular protrusion. However, the tear-drop was not visible or moved medially in 10 of our patients. Thus, we adopted the method of Sotelo-Garza and Charnley [14] for the measurement. We took the rim of the original pelvis, a projection of the upper margin of pubic ramus, as a reference line instead of the tear-drop and measured the distance between the original pelvic rim and the quadrilateral plate of protruded pelvis (Fig. 1).
The amount of acetabular protrusion ranged from 2.8 mm to 21.9 mm (mean, 9.4 mm). According to the Sotelo-Garza and Charnley system [14], the grade of protrusio acetabuli was mild (< 5 mm) in 5 hips, moderate (6–15 mm) in 20 hips and severe (> 15 mm) in 2 hips.
Preoperative planning
Preoperatively, AP and trans-lateral hip radiographs, scanography, and CT scans (Mx8000 IDT; Philips, Eindhoven, The Netherlands) of the pelvis and proximal femur were taken. We used on-screen templating with digital radiographs to decide the size of the implant [15]. On the preoperative CT scan, we measured abduction and anteversion of the acetabulum to guide the cup positioning [16].
Surgical Techniques
All operations were performed by 3 high-volume (> 200 hip surgeries/year) surgeons using Kocher-Langenbeck approach [17]. In all patients, the sciatic nerve was identified and protected during the operation.
When there was a risk of posterior wall fracture of the acetabulum during the dislocation maneuver, we did not dislocate the femoral head. Instead, 2 osteotomies; the first osteotomy below the femoral head and the second one at the base of the femoral neck, were made. Then, 1.5 to 2.5 cm thick block of the femoral neck was excised, and the femoral head was removed from the acetabulum [18].
We prioritized the press-fit fixation of acetabular cup than the restoration of hip center. Acetabular preparation was performed in 2 stages. We reamed the peripheral edge of the acetabulum first and gradually increased the diameter of the reamer until we obtained a surface reamed enough to obtain a press-fit of cementless acetabular cup. After then, cartilages and fibrous tissues of the medial floor inside the acetabulum were removed.
In 18 hips, there was a medial acetabular defect after reaming, and we filled the defect with autogenous bone graft from the excised femoral head. The bone graft was firmly impacted and was rounded using reverse reaming.
The acetabular cup was positioned using the CT measurements of acetabular abduction and anteversion as the alignment-guide [16]. The target abduction of the cup was 40°-45° [19]. The target anteversion of the cup was 15° until August 2009. After then, the cup was anteverted according to the concept of combined anteversion [16, 20]. We exclusively used cementless implants, because we were concerned of cement-related cardiopulmonary complications [21].
A press-fit of the acetabular cup was obtained in 24 hips. In the remaining 3 hips, a press-fit could not be obtained because the acetabular rim defect was > 50% or the acetabular rim was too thin. Thus, reinforcement acetabular components with a hook and three iliac flanges were used in these 3 hips.
PLASMACUP® SC (Aesculap, Tuttlingen, Germany) was used in 9 hips, Bencox cup (Corentec, Seoul, South Korea) in 7 hips, and Pinnacle cup (DePuy, Warsaw, IN) in 5 hips. G7 cup (Zimmer Biomet, Warsaw, IN) in 1 hip, ABT cup (Zimmer Biomet) in 1 hip and Delta TT cup (Lima lto, Udine, Italy) in 1 hip. In 3 hips with defective rim of the acetabulum, we used SPH reinforcement cups (Lima lto, Udine, Italy).
A nonunion was found at the transverse acetabular fracture site in 1 hip with posttraumatic osteoarthritis. The nonunion was fixed with a reconstruction plate.
BiCONTACT® stem (Aesculap) was used in 10 femurs, Bencox M stem (Corentec) in 8 femurs, Corail stem (DePuy) in 4 femurs, Taperloc Microplasty (Zimmer Biomet) in 2 femurs, KAR stem (DePuy) in 1 femur, Trilock stem (DePuy) in 1 femur, and Minima stem (Lima lto) in 1 femur.
Delta ceramic-on-ceramic bearing (BIOLOX delta, CeramTec, Plochingen, Germany) was used in 16 hips, alumina ceramic-on-ceramic bearing (BIOLOX® forte, CeramTec) in 7 hips, alumina ceramic-on-polyethylene bearing in 3 hips, and a metal-on-polyethylene bearing in 1 hip. The diameter of the femoral head was 28 mm in 9 hips, 32 mm in 11 hips, and 36 mm in 7 hips.
After the implantation and reduction of the hip prostheses, the posterior capsule and the short external rotators were tightly repaired to the crest of the greater trochanter [22].
Postoperative care
Patients were encouraged to walk with toe-touch weight bearing with the aid of 2 crutches for 4 weeks and then were allowed weight-bearing.
Follow-up evaluations
Follow-up evaluations were performed at 6 weeks, 3, 6, 9 and 12 months, and every year thereafter. At each follow-up, AP and trans-lateral hip radiographs were taken and modified Harris hips score (mHHS) were measured. Postoperative scanogram was taken at 6-week follow-up.
Classification of medialized cup and restored cup
The restoration or medialization of cup center was evaluated on postoperative 6-week AP radiograph. When any portion of the acetabular cup protruded medial to the Kohler’s line, the hip was classified as medialized group. When whole portion of the cup was located lateral to the Kohler’s line, the hip was classified as restored group.
Eighteen cups; 15 primary cups and 3 reinforcement components, were classified as restored group, and 9 cups as medialized group. In the medialized group, the amount of medialization ranged from 6.8 mm to 19.6 mm (mean, 11.8 mm) (Table 2).
Table 2
Hip center restoration, intraoperative press-fit and postoperative stability of cup in 27 total hip arthroplasties of 23 patients with protrusio acetabuli
Patient
|
Sex
|
Age (year)
|
Side
|
Cause of protrusion
|
Preoperative protrusion (mm)
|
Hip center restoration
|
Intraoperative press-fit of cup
|
Cup stability
|
Follow-up duration (years)
|
#1
|
F
|
77
|
Rt.
|
Post-radiation osteonecrosis
|
11.3
|
Restored
|
Not obtained
|
Loose
|
6.4
|
#2
|
F
|
71
|
Lt.
|
Post-traumatic arthritis
|
11.7
|
Restored
|
Obtained
|
Stable
|
7.2
|
#3
|
M
|
36
|
Rt.
|
Ankylosing spondylitis
|
7.2
|
Restored
|
Obtained
|
Stable
|
5.1
|
|
|
25
|
Lt.
|
Ankylosing spondylitis
|
7.4
|
Restored
|
Obtained
|
Stable
|
16.2
|
#4
|
M
|
68
|
Rt.
|
Rheumatoid arthritis
|
8.9
|
Restored
|
Obtained
|
Stable
|
8.6
|
#5
|
F
|
60
|
Rt.
|
Rheumatoid arthritis
|
15.9
|
Restored
|
Obtained
|
Stable
|
5.4
|
#6
|
F
|
69
|
Lt.
|
Post-traumatic arthritis
|
9.9
|
Restored
|
Obtained
|
Stable
|
8.1
|
#7
|
F
|
35
|
Rt.
|
Rheumatoid arthritis
|
3.1
|
Restored
|
Obtained
|
Stable
|
2.0
|
|
|
35
|
Lt.
|
Rheumatoid arthritis
|
2.8
|
Restored
|
Obtained
|
Stable
|
2.0
|
#8
|
F
|
73
|
Lt.
|
Rheumatoid arthritis
|
4.7
|
Restored
|
Obtained
|
Stable
|
3.9
|
#9
|
M
|
67
|
Rt.
|
Ankylosing spondylitis
|
6.2
|
Restored
|
Obtained
|
Stable
|
3.1
|
#10
|
F
|
59
|
Rt.
|
Post-traumatic arthritis
|
5.3
|
Restored
|
Obtained
|
Stable
|
8.3
|
#11
|
F
|
49
|
Rt.
|
Ankylosing spondylitis
|
4.3
|
Restored
|
Obtained
|
Stable
|
2.1
|
#12
|
F
|
63
|
Rt.
|
Rheumatoid arthritis
|
8.3
|
Restored
|
Obtained
|
Stable
|
2.2
|
#13
|
M
|
62
|
Lt.
|
Previous infection
|
10.9
|
Restored
|
Obtained
|
Stable
|
2.5
|
#14
|
F
|
62
|
Lt.
|
Rheumatoid arthritis
|
4.8
|
Restored
|
Obtained
|
Stable
|
3.7
|
#15
|
F
|
74
|
Rt.
|
Rheumatoid arthritis
|
9.2
|
Restored
|
Not obtained
|
Stable
|
4.1
|
|
|
73
|
Lt.
|
Rheumatoid arthritis
|
11.3
|
Medialized (9.0mm)
|
Not obtained
|
Stable
|
4.5
|
#16
|
M
|
70
|
Lt.
|
Rheumatoid arthritis
|
13.3
|
Restored
|
Obtained
|
Stable
|
4.8
|
|
|
69
|
Rt.
|
Rheumatoid arthritis
|
14.3
|
Medialized (19.6mm)
|
Obtained
|
Stable
|
5.3
|
#17
|
F
|
66
|
Rt.
|
Post-traumatic arthritis
|
7.6
|
Medialized (8.6mm)
|
Obtained
|
Stable
|
2.3
|
#18
|
F
|
74
|
Rt.
|
Rheumatoid arthritis
|
9.0
|
Medialized (6.8mm)
|
Obtained
|
Stable
|
2.8
|
#19
|
M
|
74
|
Rt.
|
Post-traumatic arthritis
|
14.4
|
Medialized (15.8mm)
|
Obtained
|
Stable
|
3.6
|
#20
|
M
|
34
|
Rt.
|
Post-traumatic arthritis
|
12.5
|
Medialized (13.0mm)
|
Obtained
|
Stable
|
8.4
|
#21
|
F
|
55
|
Rt.
|
Rheumatoid arthritis
|
21.9
|
Medialized (11.5mm)
|
Obtained
|
Stable
|
11.7
|
#22
|
M
|
69
|
Lt.
|
Rheumatoid arthritis
|
10.4
|
Medialized (11.3mm)
|
Obtained
|
Stable
|
2.3
|
#23
|
F
|
66
|
Lt.
|
Post-traumatic arthritis
|
7.7
|
Medialized (10.1mm)
|
Obtained
|
Stable
|
5.1
|
Patients #3, #7, #15 and #16 underwent bilateral total hip arthroplasties. Patients #1 and #2 were operated with reinforcement cups
|
Cup position and radiological evaluations
The cup position was measured on postoperative 6-week radiographs. The cup abduction was measured using the method described by Engh et al. [23], and the cup anteversion using the method of Woo and Morrey [24, 25].
We evaluated postoperative leg length discrepancy, migration of the acetabular cup, the stability of the acetabular and femoral components, wear of bearing surface and osteolysis.
The leg length discrepancy was measured on postoperative 6-week scanogram [26]. We measured the vertical length between the ankle mortise and upper body of the first sacral vertebra. When the first sacral vertebra was not visualized in the scanogram, we used both sciatic notches as the proximal reference.
The 6-week AP and cross-table lateral radiographs were used as the baseline studies for the assessment of cup migration, implant stability, bearing wear and osteolysis.
The stability of acetabular cup was evaluated using the method of Latimer and Lachiewicz [27], and that of the femoral stem using the method of Engh et al. [28]. The bearing was measured according to the method by Livermore et al. [29]. A diagnosis of osteolysis was made according the criteria of Engh et al. [30]. The osteolytic lesions were located according to the 3 zones of DeLee and Charnley [31] on the acetabular side, and the 7 zones of Gruen et al. [32] on the femoral side.
Radiological evaluations were done by two independent observers who did not participate in THAs.
Clinical evaluation
Clinical evaluations were done using modified Harris hip score [33].
Comparison between the medialized group and restored group
The postoperative migration of acetabular cup, implant stability, radiological changed and modified Harris hip score at the final follow-up between the restored group and medialized group were compared.
The study design and protocol of this retrospective study were approved by the institutional review board in our hospital.