Leg lengthening and femoral-offset reduction after total hip arthroplasty: where is the problem

Abstract

Background

Restoration of femoral offset (FO) and leg length is an important goal in total hip arthroplasty (THA) as it improves functional outcome.

Purpose

To analyze whether the problem of postoperative leg lengthening and FO reduction is related to the femoral stem or acetabular cup positioning or both.

Material and Methods

Between September 2010 and April 2013, 172 patients with unilateral primary osteoarthritis treated with THA were included. Postoperative leg-length discrepancy (LLD) and global FO (summation of cup and FO) were measured by two observers using a standardized protocol for evaluation of antero-posterior plain hip radiographs. Patients with postoperative leg lengthening ≥10 mm (n = 41) or with reduced global FO >5 mm (n = 58) were further studied by comparing the stem and cup length of the operated side with the contralateral side in the lengthening group, and by comparing the stem and cup offset of the operated side with the contralateral side in the FO reduction group. We evaluated also the inter-observer and intra-observer reliability of the radiological measurements.

Results

Both observers found that leg lengthening was related to the stem positioning while FO reduction was related to the positioning of both the femoral stem and acetabular cup. Both inter-observer reliability and intra-observer reproducibility were moderate to excellent (intra-class correlation co-efficient, ICC ≥0.69).

Conclusion

Post THA leg lengthening was mainly caused by improper femoral stem positioning while global FO reduction resulted from improper positioning of both the femoral stem and the acetabular cup.

Keywords

Hip arthroplasty leg length discrepancy femoral offset plain radiographs reliability

Introduction

Total hip arthroplasty (THA) is a successful treatment option in patients with hip osteoarthritis (OA) complaining of persistent disabling pain (1). Besides relieving pain, restoration of the biomechanical forces around the hip with appropriate femoral offset (FO) and leg length is an important goal to achieve good postoperative functional outcome (2 –4). It remains controversial as to how much postoperative leg length discrepancy (LLD) and FO change are acceptable. Generally, lengthening of the operated leg ≥10 mm and FO reduction of the operated hip >5 mm should be avoided. Lengthening beyond 10 mm is associated with lower functional outcome, lower patient satisfaction, more residual hip pain, and more frequent use of walking aids (3,5 –9). FO reduction of the operated hip >5 mm was associated with lower functional outcome, probably secondary to shorter lever arm and weaker gluteal muscles (10,11).

Lengthening of the operated leg and FO reduction of the operated hip can result from improper positioning and/or sizing of the implanted stem or cup or both. There are few radiological studies addressing this issue (3,12,13) and further investigation is therefore warranted. There are different modalities used to measure LLD and FO in clinical practice. Most commonly, plain radiographs are used despite their limitations compared to computed tomography (CT).

The hypothesis that the problem was related to stem positioning was addressed in this prospective cohort study. The aim was to analyze the postoperative radiographs of THA patients with leg lengthening and FO reduction to determine whether the problem is related to femoral stem or acetabular cup positioning or both. This would help radiologists to convey this important information to orthopedic surgeons.

Material and Methods

This prospective cohort study was conducted at Sundsvall Teaching Hospital after approval by the regional ethics committee at Umeå University (Nos. 07-052 M and 12-287-32 M). Between September 2010 and April 2013, 172 patients with unilateral primary OA treated with THA were included. Informed consent was obtained from all patients. Patients with secondary OA, previous spinal, pelvic, or hip fracture or surgery were excluded. There were no other exclusion factors.

One of ten specialist orthopedic surgeons, or an assistant directly under his/her supervision, performed the operations using either a cemented Lubinus SP II system (Link, Hamburg, Germany) or uncemented CLS stem and Triology cup (Zimmer, Warsaw, IN, USA). These two prosthetic types have the same collum-center-diaphysis angle (CCD) (125° versus 126°), the same neck length (short, medium, and long), the same head size (32 mm), and the same taper measure (12/14 mm). The posterolateral approach was used in all operations. Preoperative radiological templating using the Mdesk™ system (RSA Biomedical, Umeå, Sweden) was performed in 75–80% of cases. Templating was carried out by importing the AP pelvic radiographs from the Picture Archiving and Communication System (PACS) to the Mdesk™ system program where digital templates (obtained from the manufacturer) were placed over the digital images. Templating was started by image calibration using a 30-mm radiopaque standardized metal ball. Thereafter, the following measurements were obtained: the cup size, stem size, neck length, and CCD angle of the prosthetic type intended to be used by the surgeon. The templated radiographs were then saved as Mdesk™ file in a dedicated database for further analysis.

Intraoperative assessment of leg length was done by evaluation of the soft-tissue balance and manual assessment of relative leg lengths by comparing it with the contralateral side. No specific method was used to assess the intraoperative global FO restoration.

Measurement of postoperative LLD and global FO

LLD and global FO were measured in each patient at the second postoperative day using a standardized protocol. An antero-posterior (AP) hip radiograph was required with the patient supine and both legs internally rotated 15° using a leg retainer with the X-ray beam centered on the symphysis pubis with film focus distance 115 cm (14). Visual assessment of the radiographs to check for the size of the obturator foramina bilaterally was also done to ensure no or minimal pelvic obliquity. When inequalities were detected, new radiographs were required (8 cases).

The LLD on radiographs was defined as the difference in perpendicular distance in millimeters between a line passing through the lower edge of the teardrop points to the corresponding tip of the lesser trochanter (3,15). A positive LLD value was obtained when the affected limb was longer than the contralateral side, whereas a negative value indicated the opposite. Measurements were calibrated to a 30-mm radiopaque standardized metal ball to assess the degree of magnification. A 1-mm precision scale was used. Patients whose operated leg became ≥10 mm longer compared with the contralateral side (n = 41) were further studied to investigate the amount of lengthening that was related to the stem positioning (stem length) and to the cup positioning (cup length) compared with the contralateral side. This was done using the method described by McWilliams et al. (16). The stem length at the operated side was measured as the distance between the lesser trochanter and teardrop and the cup length as the distance between the teardrop and the center of rotation. The stem and cup lengths were compared with those of the contralateral side (Fig. 1).

Fig. 1.

Postoperative AP pelvis radiograph after THA using Lubinus cemented prosthesis showing the cup length at operated side (A) and at the non-operated side (A1), as well as the stem length at operated side (B) and at the non-operated side (B1). Measurements were undertaken according to McWilliams et al. (16).

The global FO measurement of the operated side was carried out on the postoperative pelvic AP view. It was calculated by adding the distance between the longitudinal axis of the femur to the center of rotation (stem offset) and the distance from the center of rotation to a perpendicular line passing through the medial edge of the ipsilateral teardrop point of the pelvis (cup offset) (4). The measurement was repeated bilaterally to compare the FO of the operated side to the non-operated hip. A positive value was used when the FO of the operated hip was greater than the contralateral side while a negative value indicated the opposite. Patients with reduction of the global FO at the operated side ≥5 mm (n = 58) were further studied to investigate the amount of global FO reduction that was related to the stem positioning (stem offset) and to the cup positioning (cup offset) compared with the contralateral side (Fig. 2).

Fig. 2.

Postoperative AP pelvis radiograph after THA using uncemented Triology CLS prosthesis showing the stem offset at operated side (A1) and at the non-operated side (A), as well as the cup offset at operated side (B1) and at the non-operated side (B). Measurements were undertaken using the method described by Lacerf et al. (4).

To test the inter-observer reliability of the measurements on the 41 patients with leg lengthening (cup length and stem length) and 58 patients with global FO reduction (cup offset and stem offset), we compared the measurements made by a radiologist (observer 1) with the measurements made by an orthopedic surgeon (observer 2). The two observers were blinded to each other’s results. To test the intra-observer reproducibility, the observers repeated the same measurements after 8 weeks and their two measurements were compared to each other. They were blinded to their own previous measurements when repeating them.

The two observers were trained in making the measurements before starting the study and a set of five patients from the leg lengthening group and five patients from the FO reduction group were measured by the whole research group to reach an agreement between the observers about how the measurements would be conducted.

Statistical analysis

The required sample size was calculated using the method described by Pocock (17) hypothesizing that LLD and FO reduction was related to the stem positioning in two-thirds of patients. This hypothesis resulted in a required sample size between 80 and 100 patients with 5% significance and 80% power. The material was tested for normality using the Kolmogorov–Smirnov test and was found to be normally distributed.

The radiological measurements were presented as means and standard deviation (SD). A two-tailed paired student t-test was used to compare the stem length and offset and the cup length and offset with the contralateral side. The intra-class correlation coefficient (ICC) was used to evaluate the inter-observer reliability (between the two observers) and intra-observer reproducibility (comparing each observer’s first and second measurements). An ICC of 0–0.20 was considered slight, 0.21–0.40 was considered fair, 0.41–0.60 was considered moderate, 0.61–0.80 was considered substantial, and 0.81–1.00 was considered excellent (18). Statistical analysis was carried out using SPSS for Windows version 20.0 (SPSS Inc., Chicago, IL, USA) and statistical significance was set at P < 0.05.

Results

The cohort consisted of 172 patients. There were 41 patients with leg lengthening and 58 patients with FO reduction. Of these two groups, 17 patients were in common. This means that patients with lengthening <10 mm and FO reduction <5 mm (n = 90) are not included in this analysis.

Leg lengthening group

There were 41 patients (24 women, 17 men; mean age, 70.5 years; SD, 8.8) in the leg lengthening group. The mean lengthening of the operated leg was 12.8 mm (SD, 2.5). The comparison of stem length and cup length with the contralateral side by the both observers showed that the lengthening was related to stem positioning (Table 1).

Table 1.

Inter-observer reliability measured in mm for observers 1 and 2 and compared using ICC.

	Observer 1 Radiologist	Observer 2 Orthopedic surgeon	ICC
Leg lengthening
Stem length operated	57.7 (SD 10.0)	59.5 (SD 9.8)	0.82
Stem length contralateral	50.9 (SD 10.4)	50.9 (SD 10.5)	0.86
P value	0.003	0.001
Cup length operated	19.0 (SD 4.4)	18.2 (SD 4.5)	0.79
Cup length contralateral	18.9 (SD 4.1)	18.8 (SD 4.2)	0.85
P value Femoral-offset reduction	0.95	0.90
Stem offset operated	49.2 (SD 9.5)	48.9 (SD 8.8)	0.93
Stem offset contralateral	57.1 (SD 9.0)	55.1 (SD 8.0)	0.95
P value	<0.001	<0.001
Cup offset operated	35.1 (SD 4.8)	35.5 (SD 4.7)	0.90
Cup offset contralateral	39.9 (SD 5.2)	40.4 (SD 5.8)	0.85
P value	<0.001	<0.001

Femoral-offset reduction group

There were 58 patients (26 women, 32 males; mean age, 72.2 years; SD, 8.5) in the FO reduction group. The mean global FO reduction was −13.3 mm (SD, 6.2). The comparison of the stem offset and cup offset with the contralateral side by both observers showed that the FO reduction was related to both the stem and cup positioning (Table 1).

The reliability and reproducibility of the measurements

The inter-observer reliability was substantial to excellent (ICC ≥0.79) (Table 1) while the intra-observer reproducibility was moderate to excellent (ICC ≥0.69) (Tables 2 and 3).

Table 2.

Intra-observer reproducibility for observer 1 (radiologist), measuring first and second readings in mm, compared using ICC.

	First measurement	Second measurement	ICC
Leg lengthening
Stem length operated	57.7 (SD 10.0)	58.6 (SD 9.2)	0.95
Stem length contralateral	50.9 (SD 10.4)	49.3 (SD 11.2)	0.91
Cup length operated	19.0 (SD 4.4)	19.0 (SD 4.8)	0.88
Cup length contralateral	18.9 (SD 4.1)	19.7 (SD 4.2)	0.88
Femoral-offset reduction
Stem offset operated	49.2 (SD 9.5)	48.7 (SD 9.8)	0.96
Stem offset contralateral	57.1 (SD 9.0)	57.4 (SD 9.8)	0.96
Cup offset operated	35.1 (SD 4.8)	34.9 (SD 4.9)	0.92
Cup offset contralateral	39.9 (SD 5.2)	39.3 (SD 4.5)	0.92

Table 3.

Intra-observer reproducibility for observer 2 (orthopedic surgeon), measuring first and second readings in mm, compared using ICC.

	First measurement	Second measurement	ICC
Leg lengthening
Stem length operated	59.5 (SD 9.8)	55.3 (SD 8.8)	0.69
Stem length contralateral	50.9 (SD 10.5)	47.3 (SD 7.9)	0.80
Cup length operated	18.2 (SD 4.5)	18.3 (SD 4.3)	0.86
Cup length contralateral	18.8 (SD 4.2)	19.6 (SD 3.7)	0.75
Femoral-offset reduction
Stem offset operated	48.9 (SD 8.8)	50.9 (SD 9.6)	0.93
Stem offset contralateral	55.1 (SD 8.0)	54.8 (SD 8.7)	0.96
Cup offset operated	35.5 (SD 4.7)	34.8 (SD 4.0)	0.94
Cup offset contralateral	40.4 (SD 5.8)	38.8 (SD 5.0)	0.88

Discussion

Our study has shown that post-THA lengthening of the operated leg ≥10 mm compared with the contralateral side was mainly caused by improper positioning of the femoral stem, whereas a decrease of global FO ≥5 mm compared with the contralateral hip was caused by improper positioning of both the femoral stem (or improper selection of CCD angle or neck length) and acetabular cup. The radiological measurement methods used showed substantial to excellent inter-observer reliability and intra-observer reproducibility and are therefore clinically useful. We think these measurement methods are useful regardless of the prosthesis models because the measurements are based on bony marks (tear drop, lesser trochanter, and femoral axis) that are not altered or affected by the prosthetic components.

Two prospective clinical trials showed that uncorrected postoperative lengthening of the operated leg ≥10 mm compared with the contralateral side was associated with less improvement in functional outcome and more use of a shoe lift compared with patients with lengthening <10 mm (9,19). A possible explanation of this outcome is the over-tensioning of the soft tissues around the operated hip with possible biomechanical and gait disturbances. Regarding global FO changes, a reduction of global FO ≥5 mm compared with the contralateral side is associated with weaker abductor muscle strength. These results could be due to shortening of the lever arm of the operated hip with subsequent loss of abductor muscle tension and power. The results are in agreement with previous studies published on postoperative LLD and FO changes (3,5 –8,10,11,20,21). Konyves et al. (3) found that lengthening was perceived by one-third of the patients at 12 months after surgery and was associated with a worse functional outcome. Wylde et al. (5) showed that one-third of patients perceived LLD and the lengthening was associated with poorer Oxford hip score (22) and limping. In a prospective multicenter study, Beard et al. (6) showed that patients with a postoperative LLD >10 mm had a significantly worse Oxford hip score at 3-years follow-up. Röder et al. (7) also reported a negative effect of leg lengthening after THA on walking capacity, limping, and patient satisfaction, while Edeen et al. (8) found that the extent of LLD correlated with the awareness of the problem, abnormal gait, and use of a shoe raise. Cassidy et al. (10) examined the effect of FO changes on pain and function after THA and found that decreasing FO by more than 5 mm resulted in an inferior functional outcome score and reduced abductor muscle strength. Sariali et al. (11) showed that a 6–12 mm decrease in FO after THA altered the gait while Yamaguchi et al. (20) and Kiyama et al. (21) showed a negative correlation between decreasing FO and abductor muscle strength.

There are different radiological measurement methods for LLD after THA that use different bony landmarks. In the Woolson method, the inter teardrop-lesser trochanter distance is used, while in the Willamson method, the transischial line-lesser trochanter distance is used. Other bony landmarks used include center of rotation of femoral head-lesser trochanter (CFR–LT method) and center of rotation of femoral head–teardrop–lesser trochanter (CFR–T–LT) (16). These methods have been validated in the literature (16,23). The Woolson method was found to be as reliable as the ortho-roentgenogram evaluation and resulted in improved correlation with full-leg radiographs compared to the bi-ischial-lesser trochanter distance (3,15). The teardrop points have also been found to be vertically and rotationally constant despite altered pelvic tilting/rotation (24). This could minimize the effect of pelvic position in plain radiographs. Furthermore, visual assessment of the radiographs to check for the size of the obturator foramina bilaterally was also done to ensure no or minimal pelvic obliquity. When inequalities were detected, new radiographs were required. McWilliams et al. (16) studied the reliability of these four methods and found that all of them had a comparable inter-observer and intra-observer reliability but the CFR–T–LT method had the advantage of being able to distinguish between LLD caused by cup position, stem position, or both. This might help orthopedic surgeons determine the site of LLD (stem, cup, or both). Konyves et al. (3) found that femoral stem positioning was associated with 82% of lengthening. This finding is in agreement with the results of the present study. These results indicate that cup positioning has no or minimal contribution to LLD after THA. Therefore, the measurement of cup length could possibly be omitted from the LLD measurement. Indeed, this omission is common in clinical practice.

The FO is commonly measured as the radiological distance between the center of rotation of the femoral head and long axis of the femur (4). This measurement does not take into account the changes caused by a difference in positioning of the acetabular cup. The latter is usually measured separately as the distance between the center of the femoral head to a perpendicular line passing through the medial edge of the ispsilateral acetabular tear drop. This is referred to as the cup offset (25). By adding the cup offset to the FO, the global FO is achieved (4,12,26).

Both FO and global FO measurement methods have been validated in the literature (27). In this study, we measured the global FO because it took into consideration the changes caused by the implant design and the valgus/varus positioning of the stem within the femoral canal as well as the changes of the acetabular center of rotation caused by cup implantation. Dastane et al. found that cup offset is an essential factor in restoring the global FO in THA and affects the incidence of bony impingement more than the stem offset (12). This was in agreement with our result.

Preoperative templating is an important factor for optimal implant positioning. It decreases postoperative LLD and improper global FO restoration. Templating allows the surgeon to calculate several vital parameters such as hip joint bone stock, component size, expected depth of seating of the femoral component within the femoral canal and position of acetabular component, potential LLD, and optimal level of proximal resection. Most patients in our study were templated with Mdesk™ digital templating system. Bertz et al. (28) showed that this system had a good validity and reliability compared with other templating systems used in clinical practice.

The present study has some limitations. Radiographic measurements made on the pelvic AP view are susceptible to error since horizontal dimensional parameters are influenced by variation in the positioning of the pelvis and proximal femurs and the divergence of the X-ray beams (4). Although a standardized positioning protocol was used for obtaining the radiographs, patient position remains a possible source of error, for instance the femoral anteversion, which could have influenced the measurements. We required new radiographs for patients whose postoperative radiographs showed asymmetry of the pelvis and proximal femurs, e.g. inequality of the obturator foramens and trochanters and obvious varus/valgus positioning. This happened in 8/172 of cases. We do realize that some mal-positioning is inevitable and could be missed, but we chose to accept minimal variations as we do that in our routine clinical practice. Another limitation is that we did not include the body mass index (BMI) in our analysis, where patient’s height could have influenced the measurements.

A CT scan is superior to a plain radiograph in measurement of LLD and FO. Plain radiographs might also underestimate the actual change compared with a CT scan. However, this underestimation should be negligible as we calculated the difference between the operated and the contralateral side. Furthermore, plain radiographs are the commonly used method to measure FO and LLD after THA in clinical practice owing to their availability and acceptable radiation exposure. These limitations are counterbalanced by the strengths of this study, which is a prospective cohort with the required number of patients and observers. Only patients with unilateral osteoarthritis were included to improve the accuracy of measurements, as the contralateral reference hip is anatomically unchanged. The observers were from two different specialties and experiences. This would make the obtained results more generalizable and therefore applicable in routine clinical practice.

In conclusion, post-THA lengthening of the operated leg is mainly caused by improper femoral stem positioning while global FO reduction results from improper positioning of both acetabular and femoral components. We recommend the adoption in clinical practice of the used radiological measurement methods, as they showed moderate to excellent inter-observer reliability and intra-observer reproducibility and therefore have clinical relevance and practical utility.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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Leg lengthening and femoral-offset reduction after total hip arthroplasty: where is the problem – stem or cup positioning?

Abstract

Background

Purpose

Material and Methods

Results

Conclusion

Keywords

Introduction

Material and Methods

Measurement of postoperative LLD and global FO

Statistical analysis

Results

Leg lengthening group

Femoral-offset reduction group

The reliability and reproducibility of the measurements

Discussion

Footnotes

Declaration of Conflicting Interests

Funding

References