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True or Apparent Leg Length Discrepancy

Which Is a Better Predictor of Short-term Functional Outcomes After Total Hip Arthroplasty?

Nakanowatari, Tatsuya PT, MS1,2; Suzukamo, Yoshimi PhD1; Suga, Toshimitsu MD, PhD3; Okii, Akira MD4; Fujii, Genji MD, PhD5; Izumi, Shin-Ichi MD, PhD1

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Journal of Geriatric Physical Therapy: October/December 2013 - Volume 36 - Issue 4 - p 169-174
doi: 10.1519/JPT.0b013e318282d2f1
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Leg length discrepancy (LLD) after total hip arthroplasty (THA) is critical for optimized biomechanics1,2 and is associated with sciatica and chronic back pain,3,4 dislocation,5 gait disorder,6,7 and general dissatisfaction.8,9 It has recently been found that patient-perceived inequality of leg length after THA is associated with a significantly poorer functional outcome.10,11 However, in patients who reported patient-perceived inequality, radiographic analysis that was used to measure true leg length generally revealed that only 36% of these patients had LLD.11 Thus, even if patients had patient-perceived inequality, that did not mean they had true LLD confirmed by hip radiography. Therefore, it has not been clarified whether LLD after THA is a predictor of patient-perceived inequality.

Leg length discrepancy can be divided into 2 etiological groups: true LLD, defined as those who are associated with shortening of bony structures, and apparent LLD, defined as those who are the result of altered mechanics of the lower extremities.12 There have been many previous studies8,11,13 on true LLD assessed by hip radiography, but few studies on apparent LLD after THA. Apparent LLD measured by the block test14 in patients after a femoral fracture has suggested to be more relevant to patient-perceived inequality than true LLD.15 We expected a similar association in patients after THA because true LLD was mostly corrected by the operation. Whether apparent LLD was associated with the functional outcome was also unclear in previous studies. Therefore, it is necessary to investigate possible predictors of patient-perceived inequality of the leg length and functional outcome, including not only true LLD but also apparent LLD.


The purposes of this study are to describe the types of LLD after THA and to identify whether true or apparent LLD is the better predictor of patient-perceived LLD and functional outcome in the short term after THA.


Our prospective, cohort study included 68 patients who underwent primary unilateral THA between April and August 2010. The inclusion criteria were as follows: age less than 85 years, weight lower than 110 kg, ambulatory without assistance or the use of a device, without a history of psychological dysfunction (eg, depression), and no severe deformity of the spine. All patients were treated at an orthopedic hospital in Japan and were scheduled for the standard clinical rehabilitation program in which they started full-weight bearing on the operated limb the day after surgery and were discharged at 3 weeks. The operation was carried out by 3 orthopedic surgeons using a posterolateral approach. We excluded patients undergoing revision or bilateral arthroplasty, bone grafting in the acetabulum, and shortening osteotomy of the femoral shaft. The study was approved by the Ethics Committee of Kansai Medical University, and all patients gave informed consent.


All patients were asked to fill out a questionnaire, which included the visual analog scale (VAS)16 for pain at rest in the operated hip, the Japanese version of the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC)17,18 and patient-perceived inequality 2 months after THA. In the VAS, ratings were recorded on a 100-mm horizontal line, where 0 represented no pain and 100 the worst imaginable pain. The patients were instructed to select a position on the line that corresponded to their level of pain. The WOMAC has 2 components: function (17 items) and pain (5 items). Each item was measured on a 5-level Likert scale. The WOMAC pain was measured separately for each hip and for pain at motion. The pain score on the operated side was used for analyses. All scores in this scale ranged from 0 to 100, with low levels indicating severe symptoms. To obtain patient-perceived inequality of the leg length, all patients were asked whether they felt they had LLD.

Physical Performance

We measured physical performance, which included self-selected and maximum walking speed (WS) and the Timed Up and Go (TUG) Test,19 2 months after THA. These tests provide measures of functional mobility, strength, and balance, and are related to activities of daily living. To test WS, we asked subjects to walk along a 16-m straight walkway on a flat floor once at their usual speed and then twice at their maximum speed. Walking speed was measured over a 10-m distance between marks 3 and 13 m from the start of the walkway. Walking speed measurements are considered to be highly reliable in healthy participants as well as in various patient populations.20

To test the TUG, we instructed the patients to rise from an armchair with a seat height of 43 cm, walk 3 m, turn around, return to the chair, and sit down. The test was performed twice at their usual speed, and the time from rising up out of the chair to being seated again was measured. Test-retest reliability of TUG (ICC = 0.97)21 has been reported in a similar test situation along with intra- and interrater reliability (ICC = 0.99).19 For maximum WS and TUG, we permitted participants to use walking aids and used the faster result in each test.

Leg Length

We measured the degree of LLD by a block test14,22,23 for apparent LLD and radiography for true LLD preoperatively and 3 weeks postoperatively (at discharge). In the block test, the patient was asked to stand with an adjustable raise under the shorter leg until it was perceived that the leg length was corrected (using patient perception and assessment of knee joint angle) (Figure 1). The block height was then measured in increments of 5 mm. A measurement in the block test 5 mm or greater was defined as apparent LLD. Radiography of both hips in extension was performed in standard anteroposterior view by using the method by Woolson et al24 as follows: the patient was in a standing position and the lower limbs were positioned with maximum internal rotation of both hips in extension. A consistently reproducible reference point on the pelvis was obtained by drawing a line transversely through the inferior borders of the 2 acetabular teardrops. The most prominent point of the lesser trochanter was taken as the corresponding reference point on the femur. A line was drawn from the femoral reference point to a perpendicular intersection with the pelvic reference line to the nearest millimeter. The LLD index on anteroposterior radiographs was calculated as the absolute value by subtracting the distance of the operated side from that of the contralateral side. All radiographic measurements were performed by 1 investigator who was blind to the functional outcome. This method has been reported to be as reliable as orthoroentgenograms25 and reproducible, with a measurement error of ±1 mm. The value calculated from radiography was defined as true LLD originating from the hip joint. These measurements were used to classify patients into 4 LLD groups on the basis of a previous study.23

Figure 1:
The standing posture of a 60-year-old woman with the right hip arthroplasty. (A) In static standing, the right knee was slightly flexed while the pelvis was tilted downward on the operated side. (B) In placing blocks under left foot, the right knee was extended and she felt that both legs became equal lengths. LLD indicates leg length discrepancy.

At 3 weeks postoperatively, we classified LLD into 2 types, using the following criteria: true LLD, when radiography of the hip indicated a discrepancy of greater than 5 mm and apparent LLD, when the block test indicated a discrepancy of 5 mm or greater. We classified the patients on the basis of the LLD types into the following groups: a T-LLD group with true LLD only; a mixed-LLD group with both true and apparent LLD; an A-LLD group with apparent LLD only; and a no-LLD group with neither true nor apparent LLD (Figure 2).

Figure 2:
Schematic diagram showing the classification based on the measurements of radiography and the block test into the following groups: T-LLD group, true LLD only; mix-LLD group, both true and apparent LLD; A-LLD group, apparent LLD only; no-LLD group, neither true nor apparent LLD. LLD indicates leg length discrepancy.

Statistical Methods

First, to compare preoperative patient characteristics, true LLD and apparent LLD were analyzed using the chi-square test for nominal variables and 1-way analysis of variance for parametric variables between the 4 groups defined at 3 weeks after THA. To compare, the differences in VAS and WOMAC scores at 2 months postoperatively were analyzed using the 1-way analysis of variance for parametric variables between the 4 groups defined at 3 weeks after THA. Those variables showing a P value < .05 were used as moderator variables in the second analysis. Second, the comparison of the physical performances at 2 months between the 4 groups defined at 3 weeks was analyzed using analysis of covariance with the Bonferroni post hoc test. In this test, the analysis group was the independent variable, while WS and TUG were used as dependent variables and any characteristics showing a significant difference in the first analysis as moderator variables. Finally, the association between the prevalence of patient-perceived inequality of the leg length at 2 months and the 4 groups defined 3 weeks was analyzed using the chi-square test. A P value < 0.05 was considered to be significant. We used the statistical software SPSS 11.0 for Windows (SPSS Japan Inc, Tokyo).


Of the 68 initial participants, 53 completed the measurements 2 months after THA. Numbers and reasons for dropout at each stage are shown in Figure 3. The 53 patients were classified on the basis of radiographic and block test results determined at 3 weeks postoperatively as follows: 12 (23%) into the T-LLD group, 17 (32%) into the mix-LLD group, 15 (28%) into the A-LLD group, and 9 (17%) into the no-LLD group. The comparison of the details preoperatively among the 4 groups is given in Table 1. A significant difference in age was found among the 4 groups (F = 4.936, P< .018) with the participants of the no-LLD group being older than those of the other groups. The comparison of the clinical outcome 2 months after THA among all 4 groups defined at 3 weeks postoperatively is shown in Table 2. Among all 4 groups, there was a significant difference in the WOMAC pain scale scores 2 months after THA (F = 3.29, P< .03), and the no-LLD group had a lower score (severe pain) than the other groups. Age and the WOMAC pain score at 2 months after THA were used as moderator variables for analyses of functional outcomes.

Figure 3:
Diagram of the study protocol, showing the measures undertaken at each stage; questionnaire (patient-perceived inequality, Western Ontario and McMaster Universities Osteoarthritis Index, and visual analog scale); physical performance (walking speed and Timed Up and Go test); THA, total hip arthroplasty.
Table 1:
Preoperative Details and LLD of the 53 Patients in Each Group
Table 2:
Results of Visual Analog Scale and WOMAC Scores at 2 Months in Each Group

The results of WS 2 months after THA are given in Figure 4; a significant difference was observed among the 4 groups defined at 3 weeks after THA (F = 2.85, P = .05). Walking speed at 2 months postoperatively was significantly slower for mix-LLD group than for the T-LLD group (P = .05; Figure 4). The results of TUG 2 months after THA are given in Figure 5; a significant difference was observed among the 4 groups (F = 3.45, P = .02). Timed Up and Go Test at 2 months postoperatively was significantly delayed for A-LLD group than for the T-LLD group defined at 3 weeks after THA (P = .04; Figure 5).

Figure 4:
Bar chart showing the mean of walking speed at 2 months that adjusted for age and Western Ontario and McMaster Universities Osteoarthritis Index pain scores, and standard error of the mean values for in each group. There was statistically significant difference with analysis of covariance among the 4 groups and with Bonferroni post hoc test between T-LLD and mix-LLD. LLD indicates leg length discrepancy.
Figure 5:
Bar chart showing the mean of Timed Up and Go Test at 2 months that adjusted for age and Western Ontario and McMaster Universities Osteoarthritis Index pain scores, standard error of the mean values in each group. There was statistically significant difference with analysis of covariance between 4 groups and with Bonferroni post hoc test between T-LLD and A-LLD. LLD indicates leg length discrepancy.

The results of patient-perceived inequality of the leg length 2 months after THA showed that both the mix-LLD and A-LLD groups had significantly higher prevalence of patient-perceived inequality than either the T-LLD or no-LLD group (P = .001; Figure 6).

Figure 6:
Bar chart showing the percentage of perceived inequality and no perceived inequality at 2 months in each group. There was statistically significant difference with chi-square test between 4 groups (P< .01). Mix-LLD and A-LLD groups had a higher percentage of perceived inequality than T-LLD and no-LLD groups. LLD indicates leg length discrepancy.


Our study showed that patients with apparent LLD had a poorer physical performance and a higher prevalence of perceived inequality in the short term after THA than patients with true LLD alone. True LLD has often been a concern in the past, but apparent LLD has rarely been discussed after THA. We investigated both true and apparent LLD and found associations between apparent LLD and functional outcomes. Wylde et al11 reported that a small minority of patients with perceived inequality of the leg length had true LLD measured by radiography 5 to 8 years after THA. White and Dougall13 reported that true LLD was not correlated with functional outcome 6 months after THA. Thus, true LLD may have a weak relationship with functional outcome in the middle and long terms after THA. In addition, Harris et al15 reported that measurements by the block test similar to that used in our study were strongly correlated with perceived inequality and a limp in patients with femoral fracture. Koga et al22 showed that block test measurements were correlated with lateral flexibility of the lumbar spine after THA. Therefore, we suggest that the block test measurements reflected the apparent LLD resulting from pelvic obliquity due to hip contracture or scoliosis, and that apparent LLD is related to the functional outcome (perceived inequality and physical performance).

The WS of mix-LLD group and TUG of A-LLD group were slower than those of T-LLD group in this study. Those of mix-LLD and A-LLD groups were also slower than those reported in the following literature. Guedes et al26 reported that the mean of maximum WS was 1.52 ± 0.2 m/s in 23 elderly subjects (72.0 ± 6.5 years of age) after THA. Nankaku et al27 reported that the mean of TUG was 8.4 ± 4.1 seconds in ambulatory 64 patients (56.7 ± 8.1 years of age) after THA. The results of T-LLD group are similar to these data in their studies. Therefore, we are convinced of general versatility that the physical performance data have adequate usability for patients after THA.

Our study also showed that the physical performance of the no-LLD group was a little worse than that of the A-LLD and mix-LLD groups. We assume that any change in LLD pre- and postoperatively may have influenced gait improvement after THA. Tanaka et al28 reported that improvements in single-support duration indicative of gait ability were negatively correlated with changes in LLD. We considered that the no-LLD group experienced a larger degree of change in LLD and thus gait ability had still not recovered compared with other groups because both true and apparent LLDs of this group were released at 3 weeks postoperatively. Therefore, we assume that the physical performance of no-LLD group may recover in the long term.

The block test is easy and quick to administer and requires no special training. Sabharwal and Kumar14 described that the block test helped in determining the apparent LLD by using varying heights of the block to establish the additional length required for the patient to feel level. In general, methods for assessing “true” LLD after THA were used such as a tape measure examining the distance between the anterior superior iliac spine and the medial malleolus and/or the radiography. In previous studies, the true LLD was often used on a prediction to determine the necessary treatment to equalize leg length.8,11,13,29 However, the present study found that the true LLD could not predict the functional outcomes in the short term after THA. In addition, this study indicated the importance of the apparent LLD measured by the block test. Therefore, we suggested the use of a block test for assessing apparent LLD after THA.

Physical performance and patient-perceived inequality of the leg length involved in this study have reported to be associated with health-related quality of life.11,30 This suggests that interventions for apparent LLD would improve the outcome of THA. The mechanism leading to the sensation of leg length inequality after THA is explained as follows. An established contracture of soft tissues and muscles surrounding the affected joint causes an inclination of the pelvis toward the operated side of the body. After THA releasing the contracture, this pelvic inclination leads to relative shortening of the other leg, which makes it feel shorter. Sobiech et al31 reported that physiotherapy involving specific muscle strengthening techniques helped eliminate this patient-perceived inequality. Previously, the studies to relieve patient-perceived inequality have often focused on true LLD. However, we propose that both perceived inequality and physical performance can be improved by approaches for treating apparent LLD. Specific interventions for treating apparent LLD require discussion.

The association identified between the apparent LLD and the physical performance has important implications for physical therapy. It is logical that the apparent LLD would impair individual's WS and TUG. A slower WS in older adults increased hip and low-back mechanical energy expenditures.32 Persisting apparent LLD may be indicative of compensatory strategies secondary to a decline in the musculoskeletal or neuromuscular system as well as physical performance. Physical therapists should interpret improvements in apparent LLD as positively related to how their patients are likely to establish better physical performance.


Apparent LLD can be a better predictor of patient-perceived inequality and physical performance than true LLD in the short term after THA.


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apparent leg length discrepancy; perceived inequality of leg length; physical performance; total hip arthroplasty (THA); true leg length discrepancy

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