Cup Alignment Change after Screw Fixation in Total Hip Arthroplasty.

BACKGROUND: Cup malalignment increase impingement, dislocation, cup migration, and polyethylene wear. Screw fixation for enhanced stability is the preferred option in cases of doubtful primary cup stability. There have been few studies about alignment changes after screw fixation, which may be another cause of cup malalignment. This study aimed to evaluate cup alignment change after screw fixation.
MATERIALS AND METHODS: Patients undergoing imageless navigation total hip arthroplasty using screws fixation for acetabular cup were corrected. After the press-fit cup was fully seated, the cup orientation was recorded. After screws were inserted, the cup orientation was recorded again to calculate the alignment change.
RESULTS: There were 99 cases with a mean age 63.7 years (25-93). Alignment change after screw fixation was found in 73 cases (73.7%). There were 56 cases (56.6%) with inclination angle change and the mean change was 2.21° (0°-8°). The inclination angle increased in 47 cases (47.5%) with 9 cases (9.1%) increased by 5° or more and decreased in 9 cases (9.1%). There was statistically significant difference between patients using one screw and patients using two or more screws in inclination angle change, 1.56° (0°-5°) and 3.4° (0°-8°), respectively (P = 0.0039). There was statistically significant correlation between inclination angle change and number of screws (r = 0.5401, P < 0.01). There were 49 cases (49.5%) with anteversion angle change and the mean change was 1.67° (0°-5°). The anteversion angle increased in 31 cases (31.3%), decreased in 18 cases (18.2%) with two cases (2%) decreased by 5° or more. There was significant difference between patients using one screw and patients using two or more screws in anteversion angle change, 1.46° (0°-5°) and 2.21° (0°-5°), respectively (P = 0.009). There was significant correlation between anteversion angle change and number of screws (r = 0.284, P = 0.048).
CONCLUSIONS: Changes in cup alignment after screw fixation were detected in most cases. It is one possible cause of cup malalignment.

Introduction

Acetabular cup malalignment has been linked to increase impingement, dislocation, cup migration, and polyethylene wear in patients undergoing total hip arthroplasty (THA).1 Several factors lead to cup malalignment, including preoperative pelvic tilt, inaccurate pelvic position preoperatively on the operating table, intraoperative pelvic movement in lateral position, and fluctuation of cup orientation during press-fit insertion.2345

In press-fit acetabular cup, primary stability is essential for bone ingrowth and achieving secondary stability. Screw fixation for enhancing stability is the option in cases with doubtful primary stability of the cup. There have been few studies about alignment change after screw fixation, which may be the other cause of cup malalignment. Using imageless navigation, the system displays real-time changes in cup alignment after screw fixation allowing the surgeon to detect and correct it. The objective of this study was to evaluate the cup alignment change after screw fixation, as one possible cause of cup malalignment and to define any influential factors including number of screws and patients’ age.

Materials and Methods

Institutional Review Board approval was obtained for the present study (099/2017). Between September 2016 and October 2017, patients undergoing imageless navigation THA using supplemental screws fixation for acetabular cup were included in this single-center study.

All cases underwent THA (Plasmafit cup and Metha or Excia stems; B. Braun Aesculap, Tuttlingen, Germany) with imageless navigation in semilateral decubitus position with OrthoPilot THA Pro software (Aesculap AG) that already had proven accuracy for angular measurement [Figure 1].6 The plasmafit was a cementless cup with microporous pure titanium coating and had three holes for supplemental screws fixation. Standard reaming technique, up to designed acetabular size, was used to acquire a tight fit between the 1.5 mm-oversized cup rim and the reamed acetabular bone. Supplemental screws fixation was based on the surgeon's impression of relatively inferior resistance during press fitting.

Figure 1

The operation was performed with imageless navigation total hip arthroplasty in semilateral decubitus position

All cases were conducted by senior surgeon (YS). Two small pins were inserted into the ipsilateral iliac crest through a stab incision. The navigation tracker was attached to the pin adaptor then anterior superior iliac spine on both sides and pubic symphysis were determined and digitalized with a metal pointer to define anterior pelvic plane (APP) as the reference plane for cup inclination and anteversion angles. The surgeon performed a modified Hardinge's approach in all cases. After the femoral head was removed, the teardrop was registered as another reference point. Then, by using the trial cup, the native inclination and anteversion angles of the acetabulum were determined. During reaming, the position of the reamer was acquired by the navigation system and the surgeon was provided with real-time information about the position of the reamer (anteroposterior, medialization, and cranialization) and its orientation (inclination and anteversion angles) in relation to APP as well as the native acetabulum. After reaching the design reaming size, the final cup was inserted and the surgeon was provided with real-time information about the cup position and orientation. Cup inclination and anteversion angles in each case was aimed at 40° ± 10° and 15° ± 10°, respectively according to the Lewinnek's safe zone.7 After the press-fit cup was fully seated by hammer blows, the cup orientation was recorded in both inclination and anteversion angles. Supplemental screws were inserted, the number of screws depended on surgeon's judgment. After screw fixation, the cup handle and tracker were attached again to record the cup orientation [Figure 2]. After finishing the cup, the femoral stem was performed and the joint was reduced. Patients’ age, gender, diagnosis, cup size, and number of supplemental screws were recorded as demographic data. Cup alignment change after screw fixation in each case was calculated.

Figure 2

The pictures show the cup orientation was recorded after press-fit (a) and after screws fixation (b)

Statistical analysis

The alignment change after screw fixation was compared between one and two or more screws using the Mann–Whitney U test. Spearman's rank correlation coefficients were used to define the correlation between alignment change and number of screws and between alignment change and patient's age. A value of P < 0.05 was considered as statistically significant.

Results

There were 99 cases in this study, 37 cases were men and 62 cases were women. The mean age of patient was 63.7 years (25–93). There were 67 cases (67.7%) of femoral neck fracture, 25 cases (25.3%) of osteonecrosis of the femoral head, 4 cases (4%) of developmental dysplasia of the hip, and 3 cases (3%) of posttraumatic arthritis. The mean cup size was 50 (48–60). One supplemental screw was used in 71 cases (71.7%) and 2 or more screws in 28 cases (28.3%) [Table 1].

Table 1

Demographics data of the patients

Parameters	Values
Number of hips	99
Gender (male/female)	37/62
Mean age (years) (range)	63.7 (25-93)
Etiology (cases) (%)
Femoral neck fracture	67 (67.7)
ONFH	25 (25.3)
DDH	4 (4)
Posttraumatic	3 (3)
Mean cup size (mm)	50 (48-60)
Supplemental screw (cases) (%)
1 screw	71 (71.7)
2 or more screws	28 (28.3)

ONFH=Osteonecrosis of the femoral head, DDH=Developmental dysplasia of the hip

Alignment changes after screw fixation were found in 73 cases (73.7%). There were 56 cases (56.6%) with inclination angle change and the mean change was 2.21° (0°–8°, standard deviation [SD] 1.77). The inclination angle increased in 47 cases (47.5%) with 9 cases (9.1%) increased by 5° or more and decreased in 9 cases (9.1%) [Table 2]. There was a statistically significant difference between patients using one screw and patients using two or more screws in inclination angle change, 1.56° (0°–5°, SD 0.84) and 3.4° (0°–8°, SD 2.23), respectively (P = 0.0039) [Table 3]. There was statistically significant correlation between inclination angle change and number of screws (r = 0.5401, P < 0.01) but no significant correlation between inclination angle change and patients’ age (r = 0.1611, P = 0.235).

Table 2

Details of alignment change after screw fixation

Parameters	Values
Number (cases) (%)	73 (73.7)
Inclination change (cases) (%)	56 (56.6)
Mean inclination change (°) (range, SD)	2.21 (0-8, 1.77)
Increased inclination (cases) (%)	47 (47.5)
Increased 5° or more (cases) (%)	9 (9.1)
Decreased inclination (cases) (%)	9 (9.1)
Anteversion change (cases) (%)	49 (49.5)
Mean anteversion change (°) (range, SD)	1.67 (0-5, 1.09)
Increased anteversion (cases) (%)	31 (31.3)
Decreased anteversion (cases) (%)	18 (18.2)
Decreased 5° or more (cases) (%)	2 (2)

SD=Standard deviation

Table 3

Comparison of alignment change between 1 screw and 2 or more screws

Parameters	1 screw	2 or more screws	P
Inclination change (°) (range, SD)	1.56 (0-5, 0.84)	3.4 (0-8, 2.23)	0.0039
Anteversion change (°) (range, SD)	1.46 (0-5, 0.98)	2.21 (0-5, 1.19)	0.009

SD=Standard deviation

There were 49 cases (49.5%) with anteversion angle change and the mean change was 1.67° (0°–5°, SD 1.09). The anteversion angle increased in 31 cases (31.3%), decreased in 18 cases (18.2%) with 2 cases (2%) decreased by 5° or more [Table 2]. There was significant difference between patients using one screw and patients using two or more screws in anteversion angle change, 1.46° (0°–5°, SD 0.98) and 2.21° (0°–5°, SD 1.19), respectively (P = 0.009) [Table 3]. There was significant correlation between anteversion angle change and number of screws (r = 0.284, P = 0.048) but not significant correlation between anteversion angle change and patients’ age (r = 0.089, P = 0.5431).

Discussion

In the past two decades, there has been a tendency toward using cementless press-fit rather than cemented acetabular cups. Initial stability of the acetabular cup is necessary to achieve bony ingrowth, which predicts long term survival of the prosthesis. Supplemental screws fixation was based on the surgeon's impression of relatively inferior resistance during press fitting. Fehring et al.8 studied about initial stability of press-fit acetabular components under rotation forces, they demonstrated no significant difference in load to failure testing between 1 and 2 mm underreaming. Despite concerns about transmission of wear particles to the acetabular bone through screw tunnel and risk of injury to neurovascular structures, supplemental screw fixation was essential in some cases, especially in osteoporotic bone, inadequate rim coverage or in revision surgery with acetabular bone loss. Milne et al.9 demonstrated in vitro that polyaxial locking compression screws significantly improved construct stiffness compared with nonlocked or cancellous screws.

Many factors have been reported as the causes of cup malalignment such as preoperative pelvic tilt. Maratt et al.2 reported mean preoperative pelvic tilt was 0.6° + 7.3° (−19–17.9) with 17% having >10° of pelvic tilt on preoperative radiographs. They found that the mean change in functional anteversion was a 0.74° increase in anteversion per degree of posterior tilt while the mean change in inclination was 0.29° per degree of posterior tilt. Inaccurate pelvic position and pelvic movement during the operation were another cause of cup malalignment. Nishikubo el al.3 reported preoperative errors of pelvic position in 249 hips in the lateral decubitus position. They found that mean absolute errors were 2.94° (SD 2.92°), 2.49° (SD 2.68°), and 5.92° (SD 5.20°) in coronal, transverse, and sagittal planes, respectively. Grammatopoulos et al.4 studied pelvic movement during hip replacement and found that the mean angular movement was 9° (SD 6°). Factors influencing pelvic movement included surgeon, approach (posterior > lateral), procedure (hip resurfacing > THA), and type of support. Another possible cause of cup malalignment was misdirection from the target angle during press-fit cup insertion using hammer blows while seating the cup. Nishii et al.5 studied fluctuation of cup orientation during press-fit insertion. They reported that the mean maximum deviated inclination (MDI) and maximum deviated anteversion (MDA) were −3.7° +4.0° (−12°–10°), and 0.67° +4.0° (−8°–10°), respectively. They found 1 (1%) and 29 (41%) hips with increases and decreases in MDI of 5° or more, and 13 (19%) and 6 (8%) hips with increases and decreases in MDA of 5° or more.

Fujishiro et al.10 studied the effect of screw fixation on acetabular component alignment change using CT-based fluoroscopic matched navigation system in 144 THAs. They demonstrated that the mean intraoperative change of cup position was 1.78° +1.6° (0°–5°) for inclination and 1.81° +1.6° (0°–8°) for anteversion. The intraoperative change of anteversion correlated with the number of screws and the intraoperative change of inclination correlated with the medial hip center.

Consistent with the results of this study, 73.7% of cases demonstrated alignment change after screw fixation with mean results of 2.21° (0°–8°, SD 1.77) and 1.67° (0°–5°, SD 1.09) for inclination and anteversion angles, respectively. In most cases, the inclination angle increased. Interestingly, 9 cases (9.1%) increased 5°–8°, in which all 9 cases were femoral neck fractures, which might increase dislocation rate if the surgeon could not detect and correct the alignment when using the manual technique. The anteversion angle also increased in most cases, which might increase anterior dislocation. We found a significant correlation between alignment change and the number of screws in both inclination and anteversion. It would be of concern when the surgeon needs to insert more than one screw, the alignment might be changed to a greater extent, which may result in increasing impingement and dislocation.

There were some limitations in this study.First, the number of patients was relatively small. Furthermore, the main diagnosis in the present study was femoral neck fracture (67.7%), and the mean age was rather high, and hence, it might mean decreased bone quality for achieving initial stability of the press-fit cup. Different outcomes might be obtained according to different etiologies. However, this study had some strength. As we know the incidence of dislocation was relatively high in elderly femoral neck fracture patient undergoing THA because of decreased soft tissue tension. If the patient already had some factors leading to cup malalignment as mentioned above and the surgeon was not concerned about the alignment change after screw fixation which is usually used in these patients, it might increase the incidence of dislocation.

Conclusions

Cup alignment changes after screw fixation were detected in most cases in this study. It is one possible cause of cup malalignment which might have resulted in increasing complications of THA, especially impingement and dislocation. Cup alignment change should be evaluated in cases where supplemental screws were necessary, especially in elderly femoral neck fracture patients and imageless navigation was helpful in this situation.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form, the patients have given their consent for their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.