Severe Leg Length Discrepancy after Total Hip Arthroplasty: Where is the Problem- Cup or Stem? A Retrospective Controlled Study


 Background: The factors that contribute to the severe leg length discrepancy (LLD) after total hip arthroplasty (THA) are presently unclear. In the current study, we investigated whether the acetabular cup position and femoral stem are related to the patients with severe LLD by comparing with patients who had the leg length equality.Materials and Methods: Between June 2013 and July 2019, patients undergoing unilateral primary THA in the Department of Orthopaedics at our institution were screened. According to the postoperative leg length difference, a total of 178 patients were included in the study. A group of 107 patients had leg length equality (LLD≤3 mm) and the other group of 71 patients had severe LLD (LLD≥15 mm). A standardized protocol for evaluation of anteroposterior hip radiographs was used to measure the postoperative LLD. The difference of acetabular cup position (vertical centre of rotation (VCR), horizontal centre of rotation (HCR)) and the femoral stem(stem height; stem size; femoral neck osteotomy level) between the two groups. We also assessed whether there was a difference in proximal femur morphology between the two groups.Results: On the femoral side, the mean stem height of (70.82±5.49) mm in the severe LLD group was significantly different from the (67.35±6.88) mm mean discrepancy in the leg length equality group (P < .001). Stem size and femoral neck osteotomy level showed no statistically differences between the two groups (P>0.05). On the acetabular side, there was no significant difference between the two groups in the VCR and HCR (P>0.05) . There was also no statistical difference in proximal femur morphology between the two groups (P>0.05).Conclusion: The main reason for the occurrence of severe LLD may be due to improper treatment of the femur side. Although preoperative template could choose a better stem size and femoral neck osteotomy level, surgeons need to pay more attention to the whole operation process of the femur stem.


Introduction
Leg length discrepancy (LLD) is a common complication after total hip arthroplasty (THA) [1][2][3], especially severe LLD, may result in patients' decreased satisfaction and serious consequences. Such as gait asymmetry, lower back pain ,nerve injury and prosthesis loosening [2,4,5]. There are many factors that may affect the occurrence of postoperative LLD [6,7].However, few studies have reported that the main causes of severe LLD in patients after THA.
The our aim was to investigate the factors that contribute to postoperative severe LLD. We analyzed the factors from two aspects. One is the acetabular cup side, such as vertical centre of rotation (VCR) and horizontal centre of rotation (HCR). And the other is femoral stem side, such as stem height, stem size and femoral neck osteotomy level. Proximal femur morphology was also evaluated by femoral cortical index (FCI).

Inclusion and Exclusion Criteria
Inclusion Criteria included: ( ) Patients who underwent unilateral primary THA in the Department of Orthopaedics at our institution between June 2013 and July 2019; ( ) meet the diagnostic criteria for osteoarthritis osteonecrosis or femoral neck fracture,and have de nite indications for surgery; ( ) the related outcomes of patients were completely recorded.
Exclusion criteria included: ( ) Acetabular dysplasia or defect; ( ) contralateral hip space stenosis or fusion; ( ) history of hip surgery; ( ) proximal femoral deformity; ( ) preoperative LLD was greater than 2 cm in patients with osteoarthritis and osteonecrosis;

Group Allocations
Two groups of patients were included in this study: 107 consecutive patients with LLD no more than 3 mm, who underwent THA during June 2013 and July 2019 were de ned as leg length equality group; whereas 71 consecutive patients with LLD of 15 mm or grater were de ned as severe LLD group, who underwent THA during the same time period. Our cohort was subdivided by LLD: ≥15 mm were severe LLD: ≤3 mm were leg length equality.

Perioperative treatments
Preoperative template measurements were taken for all patients, and all operations were completed by the senior surgeons. In each operation, the rotation center and the lower limb length were restored anatomically as much as possible. Implants used were the M/L Taper (Zimmer Biomet, Warsaw, Indiana) and the LCU (Link, Hanburg, Germany).

Radiological measurement
All measurements were taken with Picture Archiving and Communication Systems (PACS). FCI was measured on the basis of anteroposterior hip radiographs taken before surgery, and other radiological data were taken at least four weeks after operation. We only measured intra-observer reliability (not interobserver reliability), and the intra-observer reliability of LLD, VCR, HCR, stem height, femoral neck osteotomy, FCI level were 0.905, 0.865, 0.872, 0.870, 0.885, 0.875.

Leg Length Discrepancy
The leg length discrepancy (LLD) refers to the difference of bilateral lower limb length after THA. LLD was de ned as the perpendicular distance between the lesser trochanters and a horizontal reference line connecting the inferioraspect of the acetabular teardrops ( Figure 1).

Vertical Centre of Rotation
Vertical centre of rotation (VCR) refers to the height at which the rotational center moves up or down after THA. After surgery, VCR was measured as the distance from the rotational center to the teardrop line ( Figure 1).

Horizontal Centre of Rotation
Horizontal centre of rotation (HCR) is the distance that the acetabular cup moves horizontally inward. It was de ned as the horizontal distance between a perpendicular line through the teardrop and the hip rotational center (Figure 1).

Stem Height
Stem height is the height of the femur stem relative to the femur after surgery. Stem height was measured as the distance from the rotational center to the lesser trochanters ( Figure 2).

Femoral Neck Osteotomy Level
Femoral neck osteotomy level refers to the osteotomy height at the neck of the femur. Scholars usually de ne the distance between the medial side of the femoral neck osteotomy and the superior level of the lesser trochanter as femoral neck osteotomy level ( Figure 2).

Femoral Cortical Index
Femoral cortical index (FCI) refers to the proximal morphology of the femur, which was de ned as the ratio of cortical width minus endosteal width to the cortical width at a level of 10 cm below the tip of the lesser trochanter on the AP radiograph ( Figure 3) [8].

Statistical Analysis
Statistical analyses were performed using SPSS Version 21 (IBM Corp., Armonk, New York, USA). All continuous variables are reported as mean and standard deviation, and an independent t-test was used to detect differences between two groups. All categorical variables are reported as number and percent, and the chi-squared test or Fisher's exact test was used. All signi cance levels were set at P =0.05.

Results
There was no difference in general results between patients with severe LLD group (N = 107) and leg length equality group (N = 71). Proximal femur morphology showed no statistical difference between the two groups(P = 0.252). (Table 1)  On the side of the femur, the mean stem height of (70.82±5.49) mm in the severe LLD group was signi cantly different from the (67.35±6.88) mm mean discrepancy in the leg length equality group (P < .001). Femoral neck osteotomy level showed no signi cant difference between the two groups ( P = 0.157). On the side of the acetabulum, there was no signi cant difference between the two groups in the VCR and HCR (P = 0.130;P = 0.308) ( Table 2). And stem size showed no statistical difference in between the two groups (Table 3).

Discussion
The LLD may lead to low back pain, joint stiffness or dislocation, nerve injury and prosthesis loosening and other complications after THA [1,9]. And this leads to patient satisfaction decreased and medical litigation [10,11]. Renkawitz et al. [12]conducted a 3D gait analysis of 60 patients undergoing THA, and found that the gait kinematics of the patients was abnormal when the postoperative LLD larger than 5 mm. Austin et al. [13]reported that it was acceptable when the difference length of lower limbs was no more than 7 mm. Friberg et al. [14] found that the occurrence of complications after THA in 1157 patients was highly correlated with the unequal length of lower limbs, and 79% of sciatica and 89% of unilateral hip symptoms occurred on the longer side of the lower limbs. The occurrence of severe leg inequality is a problem that every surgeon tries to avoid. And we should pay attention to what could avoid this situation as much as possible in the THA.
In this study, we found no signi cant difference in the surgical treatment of the acetabular side between patients with severe LLD and leg length equaity. However, in the femur side, the severe LLD patients showed the greater stem height after THA. A retrospective study involving 172 THA by Al-Amiry et al. [15] showed that the leg lengthening is mostly related to the position of femoral stem, while the reduction of global femoral offset is mostly related to the inaccurate positions of the femoral stem and the acetabular cup. And Konyves et al. [16] also found appropriate placement of the femoral component could signi cantly reduce a patient's perception of discrepancy of length. These results indicate that cup positioning has less contribution to LLD after THA.
For the femoral side, stem height difference may be due to the excessive pursuit of hip joint stability by surgeons to avoid postoperative dislocation, larger stem and oversize prosthetic heads were selected to increase the offset, resulting in the severe LLD. However, it was found in this study that all patients were able to select a better prosthesis size by preoperative template measurement, inappropriate treatment of the whole femur side may lead to severe LLD. Although some studies [17,18] have pointed out that there may be a correlation between proximal femur morphology and lower extremity inequality, in our study, there was no signi cant difference in proximal femur morphology between patients with severe LLD and leg length equality. Severe LLD is mostly associated with improper management of the whole femur side.

Limitations
There are several limitations of this study. First, this study was designed retrospectively, but it involved a relatively large sample and was highly persuasive to support the observations. Second, we only selected patients without acetabular dysplasia or defect to prevent the acetabular variability. In order to ensure the accuracy of postoperative LLD results, patients with osteoarthritis and osteonecrosis whose preoperative LLD was greater than 2 cm were excluded.

Conclusion
The main reason for the occurrence of severe LLD may be due to improper treatment of the femur side.
Although preoperative template could choose a better stem size and femoral neck osteotomy level, surgeons need to pay more attention to the whole operation process of the femur stem. ; a/a' = perpendicular distance from the lesser trochanters to Line A; b (horizontal centre of rotation)= perpendicular distance from the centre of rotation to Line B; c (vertical centre of rotation)= perpendicular distance from the centre of rotation to Line A. Leg length discrepancy = a -a'.

Figure 2
d (stem height ) = distance from the centre of rotation to the lesser trochanters; e = femoral neck osteotomy level.