An intraoperative device to restore femoral offset in total hip arthroplasty

Background Leg length discrepancy (LLD) after total hip arthroplasty (THA) can lead to unsatisfactory outcome. Our objective was to design and evaluate a simple and reliable intraoperative device (Length-offset Lever) to minimize leg length discrepancy. Methods This device was used in 51 patients undergoing primary total hip replacements. The leg length discrepancy was measured pre- and postoperatively based on plain radiographs. Results Preoperative radiographic leg length discrepancy averaged 13.5 ± 6.2 mm. Leg length discrepancy showed significant improvement, with a postoperative average of 4.1 ± 2.3 mm (p < 0.0001). There were no complications associated with this device. Conclusions The ‘Length-offset Lever’ is a useful intraoperative tool to restore anatomic femoral offset and height of femoral head.


Introduction
One of the main goals of total hip arthroplasty is to restore and optimize normal hip biomechanics. Femoral offset is defined as the perpendicular distance between the centre of femoral head and the long axis of the femoral component. Charnley [1] has highlighted the importance of femoral offset on the hip joint forces.
This paper describes a simple, reliable and safe intraoperative device, Length-offset Lever. It can be used to perform THA, done through a direct lateral approach, without requiring additional equipment, incisions, or radiographs ( Figure 1). It is a 'two-in-one' device used to measure femoral offset, intraoperatively.

Patients
In our study, preoperative diagnoses were Crowe I or II developmental dysplasia of the hip in 23 patients, osteonecrosis in 12, osteoarthritis in 10, and rheumatoid arthritis in 6. Of all the patients, 22 were male and 29 were female. This study was approved by the ethics committee of The Second Affiliated Hospital of Wenzhou Medical University, and informed consents were obtained from participants or their authorized persons.
We studied a total of 51 patients undergoing primary THA between March 2008 and December 2011, using the Length-offset Lever. Revisions, THAs requiring osteotomies, and first operations of staged bilateral THAs were excluded. All THAs in this study were performed with cementless fixation utilizing direct lateral approach.

Surgical technique
Fifty-one patients were placed in the lateral position and direct lateral approach to the hip was used. Length-offset Lever consisted of a 'base' and 'measuring ruler'. After dislocation of the hip and prior to the femoral neck cut, we determined the center of the femoral head with the following procedures: first, we let compasses be perpendicular to the plane of the femoral head; then we find three points on the boundary of the femoral head and the center is equidistant to three points by compasses ( Figure 2), the 'base' unit was fixed to the anterior aspect of the greater trochanter using K-wires. The anatomic femoral offset was measured after sliding the ruler's tip to the centre of the femoral head ( Figure 3A). The 'ruler' was then removed, leaving the 'base' alone. After inserting the trial components, the 'ruler' was reinserted into the 'base', and the new offset of the femoral head was then measured ( Figure 3B). If we planned to lengthen the leg by 10 mm, we can have the femoral head trial 10 mm above the tip of the ruler. The fovea should be cleared of soft tissue to evaluate acetabular depth, and transverse acetabular ligament can be used to confirm orientation and height of the acetabular component.

Postoperative management
Leg length discrepancy was evaluated from an anteroposterior radiograph of the pelvis with both hips in neutral position (Figure 4). Radiographs were recorded on PACS (Picture Archiving & Communication System, INFINITT Co., Ltd, Guro-gu, Seoul, Korea) with a resolution of 1,536 × 2,048 pixels. A horizontal line was drawn through the bottom of the ischial tuberosities. Discrepancy in leg length was measured using the relationship between this line and the most prominent point of the lesser trochanter.
Clinical evaluations were based on the Harris hip scores [12]. Student's t tests were performed to determine statistical significance.
The postoperative average of femoral offset compared to the contralateral normal one on radiographs was recorded on PACS. Statistical analysis of the measurements was conducted using paired t tests, setting the level of significance at 0.05.

Results
Mean age of patients was 65.59 years (range 18 to 84). The average follow-up period was 18 months (range 12 to 30). Preoperative radiographic leg length inequality averaged 13.5 ± 6.2 mm (range +33 to −12 mm). Leg length discrepancy showed significant improvement, with a postoperative average of 4.1 ± 2.3 mm (range 0 to 5) (p < 0.0001). None of the patients required shoe lifts for equalization of leg lengths nor complained of   leg length discrepancy, postoperatively. No complications associated with Length-offset Lever were observed in these patients. There were no complications of dislocation, infection, fracture, or sciatic nerve palsy.
Harris hip scores significantly improved postoperatively. They increased from 45.9 ± 14.2 points preoperatively to 83.3 ± 12.6 points at the latest follow-up (p < 0.0001).
The paired t test comparisons showed statistically no significance difference between postoperative average of femoral offset and the contralateral normal one (p > 0.50).

Discussion
It can be a difficult task to maintain hip stability while trying to achieve equal leg length during THA [13][14][15][16][17]. A longer neck length is often used to improve the stability of THA. With modern femoral component designs, surgeons have more options to balance the leg length while maintaining hip stability. One example is the modular femoral neck component, which allows for flexible adjustment of femoral offset and version.
Konyves and Bannister [2] found that patients with lengthened operative legs had poorer Oxford hip scores than those that were shorter or had equal leg lengths. They believed that LLD was most commonly caused by over-lengthening of the prosthetic head-neck distance. The accuracy of femoral offset measurement using preoperative plain radiographs is affected by the rotation of the femur and pelvic tilt [18,19]. Surgeons, in the past, have used methods or devices like pins, rulers, and calipers for intraoperative measurement of leg length discrepancy [8,10,20,21]. It can be employed easily for intraoperative limb length measurement during THA, without creating a separate incision. The average leg length discrepancy postoperative was 4.2 mm in our study. No complications associated with the use of this device were observed. None of the patients expressed dissatisfaction about limb-length inequality after surgery. Safety and effectiveness of the Length-offset Lever was demonstrated in our study.
The device is effective when the leg length discrepancy is secondary to femoral deformity with minimal acetabular bone loss. Its use may not be appropriate for cases with significant acetabular dysplasia and this device does not measure the overall leg length or global offset.
There are many methods to restore femoral offset to eliminate the leg length discrepancy. For example, preoperative templating is mentioned as a common method for many surgeons which is an indirect method, its result is affected by radiographic magnification and position of patients when they had X-ray, templating overlays enlarged by a uniform factor may not be perfectly accurate. The shuck test and the dropkick test both rely on soft tissue tension as a surrogate indicator of limb length, motor blockade from spinal or epidural anesthesia may cause them to be less reliable than standard general anesthesia [22]. Surgeons also should not rely solely on the intraoperative leg-to-leg comparison because it is limited by greater adduction on the operative side and inaccuracy of palpation of landmarks through surgical drapes. Another method [23] was performed with procedures of stitching a suture into the skin superior to the surgical field and drilling a pin into the ilium bicortically, superior to the acetabulum and perpendicular to the table. It had the drawbacks of the potential loosening of the pin and positional variation of the hip during measurement. Our tool has many advantages, such as ease of use, low cost, unaffected by soft tissue tension, and measurement of the femoral offset directly and intraoperatively.
In conclusion, the Length-offset Lever is a useful intraoperative tool to restore anatomic femoral offset and height of femoral head.