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The effect of acetabular component inclination on early dislocation rate of hip arthroplasty

Firouz Madadi MD, Seyyed Reza Aghapour MD, Ali Akbar Esmaeeliejah MD, Seyyed Mohammad Qoreishy MD*, Hamid Reza Seyyed Hosseinzadeh MD

Shahid Beheshti University of Medical Sciences, Tehran, Iran

* Corresponding Author

Vol 1, Num 2, October 2014




Introduction: Dislocation after total hip arthroplasty may have many underlying reasons. Abnormal acetabular cup orientation is a major reason for this to happen. We have assessed this factor in our patients to determine how important it is.

Materials and Methods: In this descriptive cross sectional study, all cases of total hip arthroplasties performed in a 5-year interval in one educational hospital in Tehran were assessed for dislocation during their postoperative follow-up. The inclination angle of the acetabular components were measured and the relationship between dislocation and gender, age and this angle was evaluated.

Results: 441 THA's in 363 patients entered the study. 7 patients (1.5%) had dislocation in the follow-up period, all of whom had an inclination angle of over 50 degrees. Gender did not seem to affect the dislocation rate, older age, however, may probably play an important role as a risk factor for dislocation following THA.

Conclusion: Inclination angle of greater than 50 degrees in the acetabular component must be considered as a risk factor for dislocation following THA.

Keywords: Arthroplasty, Hip, Acetabular component, Dislocation




All the orthopaedic surgeons fear from encountering complications of the total hip arthroplasties (THA). Dislocation following THA is a common and devastating complication of THA.(1) Most studies show a dislocation rate of 2-3% (0.8-10%) following THA.(1-4) This rate increases significantly in revision THA, sometimes up to 20%.(5-6)

Factors such as advanced age, previous hip surgery, concomitant neurologic deficit, alcohol abuse and nonunion of greater trochanter all have been considered as risk factor for dislocation after THA. Any factor leading to imbalance in the soft tissue of the hip joint can increase the risk of dislocation. THA performed through posterior approach and by an inexperienced surgeon has a greater risk for dislocation. However, the most common risk factor for dislocation is component malposition, esp. acetabular side. Many authors agree that positioning the acetabular component in excessive anteversion or retroversion predisposes to dislocation. (1,4,7-11)

There are few studies evaluating the effect of cup inclination angle on THA dislocation and they could not confirm or rule out its effect.(4,13)

Materials and Methods

This is a cross-sectional study on those patients who underwent THA in a 5-year period (march 2007-2012) in Akhtar orthopaedic hospital, Tehran, Iran. During this time period, 470 patients entered the study, 29 of whom did not return for follow-up and were excluded from the study. So we had 441 who committed to follow up. Their average follow-up time was 27.3 months (9 – 65 months). All the surgeries were performed by two surgeons (HRSH and AAE) who had enough experience in THA. All the surgeries were performed through modified Hardinge approach in lateral position. All the patients had preoperative and postoperative anteroposterior standard pelvic X-rays. (in a standard AP pelvic X-ray coccyx must be in line with symphysis pubis and the distance between sacrococcygeal joint and the superior border of symphysis pubis must be 2 inches)

Follow-up X-rays were taken 6 weeks, 3, 6, 12 months then annually postoperatively. We measured the acetabular inclination angle on the first postoperative X-ray between a line connecting the inferior and superolateral borders of the cup and the horizontal line.(4)

The dislocation rate and parameters such as the age and sex of the patients were recorded. We analyzed the data by the last version of SPSS.


441 hips with THA in 363 patients entered the study. 285 patients had unilateral THA and 78 cases had bilateral THA in two separate operating sessions. Among these patients, 207 patients (56.8%) were male and 157 (43.2%) were female. The average age of the patients was 46.42±14.4 years (16–72). Acetabular inclination angle was between 35 and 57 degrees. Femoral head size used in 325 patients (73.7%) was large diameter (larger than 40 mm), in 109 patients (24.7%) was 28 and 40 mm and in the rest (7 patients) was 22 mm. All the patients had a relatively similar (as far as possible) postoperative rehabilitation program.

We used no abduction pillow postoperatively and allowed the patients to sit vertically the night after surgery. All the patients were forced to become out-of-bed the day after the operation and allowed to bear weight on the operated hip as tolerated. As soon as the abductor force was measured to be ⅘, the walking support was eliminated and the patient was allowed to bear weight fully on the operated limb.

We had 7 cases of dislocation, all of which occurred in the first 3 months post surgery. 4 of them were female and 3 were male. 6 of these dislocations were posterior and only one was anterior.

In all the patients with dislocation, the acetabular inclination angle was more than 50 degrees, but among 434 hips without dislocation, only 12 cases (2.7%) had acetabular inclination angle over 50 degrees. Six out of 7 patients with dislocation were older than 60 years and only one of them was 23 years old, which may show that age may be a predisposing factor for dislocation. Although the sex ratio in dislocation and non-dislocation groups was reverse, there was no statistically significant relationship between sex and dislocation rate.


One of the most important and devastating complications following THA is dislocation, esp. if recurrent, it will disable the patient.(4) The reported rate for this complication in the literature is 0.8 - 10%.(15,16)

In the study by Yuan et al, sex and age were not shown to be a risk factor for dislocation.(4) Woo et al reported the dislocation to be more prevalent in women than men.(17) many researches have assessed the effect of advanced age on dislocation after THA.(13,18) Woolson et al showed that 57% of those who had dislocation were older than 70 years.(19) In our study, we could not find any statistically significant relationship between sex and dislocation rate, but there was this relationship between advanced age and dislocation rate.

Although there are several reasons for dislocation after THA, the most important one has been mention in the literature to be acetabular component position. Both version and inclination angles are important but version has gained more attention and there are only few studies assessing the effect of cup inclination on dislocation rate. According to Hirakawa et al the long-term results of THA in patients with acetabular inclination angle ≤40 degrees are better than those with angle >40 degrees. They also showed that more than 90% of those in whom this angle was ≥45 degrees had mechanical failure of the prosthesis after 15 years.(20)

In the study by Yuan et al, they showed that the acetabular inclination angle had a direct relationship with dislocation rate.(4) Biedermann R et al. found that the acetabular position was a major determinant for dislocation. They also showed that excessive acetabular inclination angle is a risk factor for dislocation.(1) Von Knoch M et al reported that acetabular cup malpositioning is the major and most common risk factor for dislocation.(21)

Lewinnek GE et al showed that the best position for acetabular cup is in the range of 40 ± 10 degrees of inclination.(3)

McCollum DE et al reported the best inclination angle for acetabular cup to be 30–50 degrees to prevent impingement and dislocation.(5)

Ali Khan MA et al considered inclination angle >50 degrees too vertical position and showed that this too vertical position is a major risk factor for dislocation.(6)

Paterno SA et al could not find any relationship between inclination and dislocation rate. Among 32 dislocations they found 30 patients with dislocation had inclination angle of 30-50 degrees. So they concluded that considering inclination angle as a major risk factor for dislocation is too much.(13)

In our study, all 7 patients with dislocation had inclination angle of >50 degrees. But, among 434 hips without dislocation, only 12 hips (2.7%) had acetabular inclination angle over 50 degrees and the rest had inclination less than 50 degrees. This study shows that excessive acetabular inclination angle has a strong relationship with dislocation after THA, those who have angles greater than 50 degrees are at greater risk of dislocation. So, surgeon experience in performing THA and inserting the cup accurately with correct version and inclination reduces the risk of postoperative dislocation. It must be emphasized that this factor is one of the several factors responsible for dislocation. Maybe it is not the most important one and other risk factors such as acetabular version, femoral stem version, head size, head-neck ratio, neck geometry, hip soft tissue condition, history of previous hip surgeries and rehabilitation protocol all are factors affecting the dislocation rate.


Firouz Madadi MD
Associate professor, Orthopaedic surgeon, Shahid Beheshti Medical University, Tehran, Iran


Seyyed Reza Aghapour MD
Orthopaedic surgeon, Shahid Beheshti Medical University, Tehran, Iran


Ali Akbar Esmaeeliejah MD
Associate professor, Orthopaedic surgeon, Shahid Beheshti Medical University, Tehran, Iran


Mohammad Qoreishi MD
Orthopaedic surgeon, Assistant professor, Shahid Beheshti Medical University, Tehran, Iran


Hamid Reza Seyyed Hosseinzadeh MD
Orthopaedic surgeon, Associate professor, Shahid Beheshti Medical Univerity, Tehran, Iran


None declared.


Financial disclosure:
None declared.



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