Reliability study of measuring leg length discrepancy in patients with total hip arthroplasty using the tip of the greater trochanter

Background Although the tip of the greater trochanter is widely used in THA to adjust leg length discrepancy (LLD), its accuracy and reliability are still unknown. The aim of this study was to investigate the reliability of measuring LLD in patients with total hip arthroplasty using the distance from the tip of the greater trochanter to the inter-teardrop line Methods 128 patients who underwent THA in our department with preoperative and postoperative bilateral hips anteroposterior(AP) radiographs were reviewed. The distance between the bilateral anterior superior iliac spine and the medial malleolus (ASIS-MM) was measured before and after the operation. The distances from the vertex of the lesser trochanter to the bi-ischial line(LT-BI), the distance from the vertex of the lesser trochanter to the inter-teardrop line(LT-IT) and the distance from the tip of greater trochanter to the inter-teardrop line(GT-IT) were measured on the bilateral hips AP radiograph preoperatively and postoperatively. Taking X-ray magnification into account when measuring data. Use one-way ANOVA and paired t test to compare the differences between the four methods. The agreements between the four methods was evaluated using Bland-Altman analysis.


Background
Although the tip of the greater trochanter is widely used in THA to adjust leg length discrepancy (LLD), its accuracy and reliability are still unknown. The aim of this study was to investigate the reliability of measuring LLD in patients with total hip arthroplasty using the distance from the tip of the greater trochanter to the inter-teardrop line Methods 128 patients who underwent THA in our department with preoperative and postoperative bilateral hips anteroposterior(AP) radiographs were reviewed. The distance between the bilateral anterior superior iliac spine and the medial malleolus (ASIS-MM) was measured before and after the operation.
The distances from the vertex of the lesser trochanter to the bi-ischial line(LT-BI), the distance from the vertex of the lesser trochanter to the inter-teardrop line(LT-IT) and the distance from the tip of greater trochanter to the inter-teardrop line(GT-IT) were measured on the bilateral hips AP radiograph preoperatively and postoperatively. Taking X-ray magnification into account when measuring data.
Use one-way ANOVA and paired t test to compare the differences between the four methods. The agreements between the four methods was evaluated using Bland-Altman analysis.

Results
There was no statistical difference between the four methods. Comparing GT-IT with MM-ASIS, ,LT-IT and LT-BI, the preoperative P values were 0.369, 0.581, and 0.402, and the postoperative P values were 0.755, 0.502 and 0.233. Comparing LT-BI with MM-ASIS and LT-IT, the P values were 0.151 and 0.372 preoperatively, and 0.179 and 0.917 postoperatively. Comparing LT-IT with MM-ASIS, the P value was 0.924 before surgery and 0.291 after surgery. Bland-Altman indicated that there were good agreements between GT-IT and LT-IT.

Conclusion
It is highly reliable to evaluate the LLD of patients with THA by using the difference in the distance between the tip of the greater trochanter and the inter-teardrop line.

Background 3
Leg length discrepancy (LLD) caused by total hip arthroplasty can produce a variety of complications, such as sciatic nerve paralysis [1], gait abnormality [1,2], hip -spine syndrome [1], and recurrent hip dislocations [3]. Therefore, the difference in the length of the lower limbs should be fully evaluated before surgery in order to properly balance the length of the lower limbs during surgery. There are currently many methods to evaluate the difference in the length of the lower limbs, such as physical measurement of the length of the lower limbs, X-ray measurement, length balance of the lower limbs with the heel pad, etc. At present, the most commonly used method in clinical practice is to measure the distance from the vertex of the lesser trochanter to the bi-ischial line or the distance from the vertex of the lesser trochanter to the inter-teardrop line on anteroposterior pelvic or bilateral hip joints radiograph. The difference in bilateral distance is considered to be LLD of both lower limbs [4].
This study evaluated the LLD by measuring the distance between the tip of the greater trochanter and the inter-teardrop line on the anteroposterior radiograph of both hips, and compared this method with the methods commonly used in the literature to evaluate its reliability.

Materials And Methods
As per Altman nomogram, using a standardized difference of 0.5 with a power of 80% and a P-value of 0.05 the sample size required for this study was 128. We measured the patients who needed to undergo total hip replacement in the osteonecrosis and joint reconstruction ward of Xi'an Honghui Hospital from August 2015 to March 2016.
AP radiographic views of the pelvis were obtained in the same radiology unit based on a routine protocol at a standard distance of 1 m and centered 2 cm above the pubic symphysis. The patients were placed in supine position with their feet rotated internally 15° to maintain neutral hip position.
The sizes of the lesser trochanters were similar and the anterior and posterior borders of the greater trochanters were superimposed on both sides [5]. The coccyx was in line with the pubic symphysis to prevent pelvic rotation in the transverse plane [6]. The distance between the sacrococcygeal joint and the pubic symphysis was 1-4 cm in men and 4-6 cm in women to exclude abnormal pelvic tilting in the sagittal plane [6,7]. Exclusion criteria: 1. Unclear teardrops; 2. Pelvic rotation or tilt; 3. Hip flexion contracture before or after surgery, cannot be straightened; 4. Other diseases of the lower limbs lead to shortening of the limbs or knee disease resulting in knee joints varus, valgus or flexion contracture deformity.
The following measures are given to eligible patients after admission: Use a medical tape to measure the distance from the left and right anterior superior iliac spine(ASIS) to the ipsilateral medial malleolus(MM).
Use the marker on AP radiograph of the bilateral hip joint to mark the following (Figs. 1 and 2).1.
Made perpendicular lines from GT to IT, and draw lines from LT to IT and BI, and measured the length of each vertical segment. Calculated the difference between the left and right sides, and the data were divided into 4 groups, the difference between GT to IT formed GT-IT group, the difference between ASIS to MM formed ASIS-MM group, the difference between LT to IT formed LT-IT group, and the difference between LT to BI formed LT-BI group. The postoperative radiograph measurement ÷ magnification1 was the actual value, and the preoperative radiograph measurement ÷ magnification 2 was the actual value. The difference in the length of the bilateral vertical line segments can be considered as LLD.

Magnification1=
Measured To determine intra-and interobserver variability, 20 randomly selected patients were measured initially by Observer I (Lu) and Observer II(Yuan). After 3 weeks the radiographs were measured again by the same authors except physical measurements. An intraclass correlation coeffcient (ICC) > 0.8 was considered excellent agreement, ICC 0.6-0.8 was fair to good agreement, and ICC < 0.6 was poor agreement.

Statistical analysis
Quantitative data are expressed as means ± standard deviation (SD). Statistical analyses were performed using PASW statistics 18 (SPSS Inc., Chicago, IL, USA). The normality assumption of our data was checked using the Kolmogorov-Smirnov test. The One-way ANOVA test and paired t test were used to detect the measurements of LLD of the lower limbs between GT-IT group and the other three groups. The Bland-Altman diagram was used to check the agreement of the four measurements. P < 0.05 was considered statistically significant.

Results
All measurements were normally distributed. The ICC values of the four groups were all greater than 0.80, which showed excellent agreement in the measurements (Table 1). The results of the four pre-and post-operative measurements are shown in Table 2. One-way ANOVA analysis showed, there were no statistically significant difference between the LLDs Bland-Altman analysis indicated that the means bias close to 0 mm between GT-IT and the other measurements( Fig. 3A-F), SD between GT-IT and LT-IT was significantly smaller than the standard deviation between GT-IT and other groups, and it has the smallest limits of agreement(LoA) which were preoperative − 13.04 and 12.59 (Fig. 3B), and postoperative − 15 and 14.84 (Fig. 3E). We set the clinical LoA of -10 mm to 10 mm. It was suggested that 87.5% and 83.5% of the measurements are within clinical LoA before and after surgery. The measurements between GT-IT and the other two methods within LoA are less than 80%.

Discussion
Many studies believe that the complications increases significantly when the LLD was greater than 1 cm after THA, so it is recommended that LLD should be less than 1cm [8]. In order to prevent LLD, it is important to correctly measure the length of the lower limbs before surgery. Measurement on radiograph of the pelvis or bilateral hip joints is one of the most commonly used methods for assessing the length of the lower limbs. Standardized AP radiograph of the pelvis or bilateral hip joints were mainly used, the pelvic side use the ischia tuberosity line or the inter-teardrop line as references [9,10], and the femoral side use the lesser trochanter [9,10]. The difference between the distance from the vertex of the lesser trochanter to the inter-teardrop line or the vertex of the lesser trochanter to the bi-ischial line can be defined as LLD. It was also reported using the center and the higher point [11] of the femoral head as a reference to the femoral side. However, the reliability of these methods were not yet consistent. At present, the distance from the vertex of the lesser trochanter to the inter-teardrop line is considered to be more accurate than the distance from the vertex of the lesser trochanter to the ischial tuberosity line [4,10,12], because the inter-teardrop line is more stable on the AP pelvic X-ray than the ischial tuberosity line, and was less affected by body position, and it is even better than the physical measurement of the distance from the anterior superior iliac spine to the medial malleolus. Meermans et al. [4] found that using the distance from the center of the femoral head to the vertex of the lesser trochanter to evaluate LLD was also a reliable method.
There can largely restore the anatomical center of rotation of the acetabulum [13]. Therefore, using this method to measure LLD can not only be used to evaluate the difference in the length of the lower limbs before and after surgery, and to guide the preoperative template, but also to directly adjust the length of the limbs using this marker during the operation.
The shortcomings of this study are: 1. The sample size is small and only 128 patients are selected; 2.
Due to the limitations of the hospital, the measurement method is manual measurement, X-ray measurement was accurate to millimeters, and physical measurement was only accurate to 5 mm; 3.
This study was not a typical prospective study, and no blind method was used. The researchers participated in the design and research. Therefore, it is still necessary to further use large samples, digital measurement, and strict use of blind research to further improve the accuracy and reliability of the research.
In conclusion, this study calculated the LLD of patients by measuring the difference in distance

Consent to publish
The participants in this study provided their written informed consent to undergo the study and to have their data used in the study.