Surgeons aim to minimize the occurrence of leg length discrepancy (LLD) following total hip arthroplasty (THA); however, prosthesis fixation and hip stability are the paramount goals. Unfortunately, patients with LLD are often angry with their surgeon and unhappy with an otherwise technically good result. It can become a difficult, self-propagating, and destructive situation, and the surgeon-patient relationship can become strained, resulting in litigation.1-7
The purpose of this article is to:
- Review the key technical aspects of the THA procedure that can minimize the occurrence of LLD
- Develop a strategy for the surgeon to evaluate and correct leg length discrepancy should it occur
Incidence of Preoperative LLD
The incidence of LLD discrepancy in the asymptomatic normal population has been reported to be as high as 90%.8,9 A study by Knutson analyzed the LLD in subjects with low-back pain, athletes, and randomly selected volunteers.10-19 LLD ranged from 0 mm to 20 mm, with a mean LLD of 5.1 mm (SD 3.9) for subjects with kinetic chain (knee, hip) problems or back pain, and a mean LLD of 5.2 mm (SD 4.2) for those with no pain. Knutson found no significance between LLD and any factors in these groups.19
In patients undergoing THA on the short side, loss of the articular cartilage in the hip has led to superior migration of the femoral head, requiring the surgeon to lengthen the affected limb.2,5,20-22 Patients can also have a long affected limb prior to THA, as shown in a study by Ranawat et al, in which preoperative LLD ranged from a 24 mm short affected limb to a 5 mm long affected limb (mean, 4.04 mm short).23
Incidence of Postoperative LLD
The average LLD following THA has been reported to be from 3 mm to as high as 16 mm.6,20,21,23-25 In a study conducted at the Rothman Institute, in which the direct lateral approach in the supine position was used, the postoperative LLD was less than 5 mm in 75%, 5-7 mm in 12.5%, and 7-10 mm in 2% of patients.1 Williamson et al found that the vast majority of patients with functional LLD (144/150 – 96%) had a long operated limb.26
Although LLD is detectable by up to 35% of patients after THA,2 the level of LLD which is tolerable for patients is still debated. Generally, it has been agreed that LLD of less than 10 mm is bearable for the patient,27 but it has also been reported that an LLD of 35 mm may not affect clinical results following THA.28 If leg lengthening causes pain or functional impairment, however, litigation may follow.3,23
Functional versus True LLD
LLD following THA can be broken down into two main components: true and functional LLD. 20,29
- True LLD is caused by lengthening of the prosthetic head-neck distance.20 This discrepancy can be measured on full-length or pelvic anterior-posterior (AP) weight-bearing radiographs. A line drawn from the center of the femoral head to the ankle center represents the true leg length on full-length AP radiographs.30 On AP pelvic radiographs, a vertical line drawn from the interteardrop line to the lesser trochanter of each side can be compared to assess discrepancy. The AP pelvis view will not account for deformities below the lesser trochanter; however, but this method has been found to be the most reliable in assessing true LLD.4,5 During the physical exam, true LLD can also be measured by comparing the position of the medial malleoli of the patient in the supine position.
- The second measurement is the functional LLD,20,31 which is the LLD that is attributable to factors such as the tightness of the anterolateral soft tissues about the hip and degenerative disease with scoliosis of the lumbar spine, causing obliquity of the pelvis.31 Functional LLD is typically assessed when the patient is standing and feels a sense of imbalance.20 To measure this discrepancy, measuring blocks are placed under the short limb until the legs feel equal.3
Consequences of Postoperative LLD
Morbidities such as balance problems, back pain, gait disturbance, generalized “hip” pain, the need for a shoe lift, nerve pain, and paresis can all be experienced by patients with LLD.7,27,32-40 The patient’s perception is that the surgeon has done something wrong, when in fact, from a technical perspective the THA may have been done well. Oftentimes, the only thing the surgeon has not done well is to discuss the possibility of limb length discrepancy with the patient preoperatively.
Strategies to Minimize LLD
Change in leg length primarily depends on an accurate measurement of the preoperative leg length discrepancy and having an accurate technique of documenting the appropriate change in leg length during the surgical procedure. Multiple techniques have been employed based on fixed reference points.
Proper component position and orientation maximizes the potential for leg length equality. Patient issues such as soft tissue laxity can create a conflict between leg length equality and hip stability. In these situations, it is imperative for the surgeon to leave the operating room with a stable hip – leg lengthening may be necessary to achieve this primary goal.
Techniques have been described that use the patient’s bony and soft tissue anatomy for visual referencing intraoperatively to assist in more reliably positioning the components.41-44 To take advantage of the patient’s anatomy intraoperatively, an adequate exposure of the bony and soft tissue landmarks must first be obtained. Goals of an ideal referencing system should be to:
- Improve the accuracy of component position
- Minimize errors of leg length
- Eliminate instability
- Maximize range of motion (ROM)
- Minimize component impingement
- Improve hip mechanics and functionality
Concerns involving the acetabular component include cup position, cup version, and cup tilt (abduction angle).44 Landmarks used for the acetabular component are various aspects of the acetabular and pelvic anatomy, which can be identified radiographically and intraoperatively.
The most important radiographic landmark is the teardrop reference point, which is a guide to the inferior acetabulum.44 This is important in restoring leg length, as well as optimizing the hip center of rotation and minimizing impingement of the components. Osteophytes and soft tissue may cover the teardrop and need to be removed at surgery to properly locate it. The position of the inferior aspect of the acetabular component with respect to the teardrop can help the surgeon calculate the abduction angle of the cup.7
Intraoperative landmarks include the superior, anterior, and posterior rims of the acetabulum; the sciatic notch; and the transverse acetabular ligament. The superior rim of the acetabulum is a guide to the abduction angle of the cup.44 The anterior and posterior rims of the acetabulum are useful as guides to the anteroposterior positioning of the cup, as well as providing a sense of the anteversion. It is important to remove any osteophytes that may hamper good anteroposterior fit of the cup or that might cause the cup to be positioned in relative retroversion if they are thought to be part of the anterior or posterior column. Archbold detailed a method of using the bony rims of the acetabulum to assess component version and position. 41
The sciatic notch is used to define the notch acetabular angle according to D’Antonio’s technique.45 This angle is defined as the angle created at the intersection of a line from the sciatic notch to another line drawn between the posterior and anterior acetabular rims. The angle was determined to be 89.0° ± 3.5° in a study of 200 cadaveric acetabulae, and thus is a reproducible method of reproducing a patient’s own anatomic acetabular anteversion. In the technique described using this method, the surgeon palpates with the index finger from the posterior acetabular wall to the greater sciatic notch; reaming parallel to this line would then recreate the patient’s own acetabular version. If greater anteversion was desired, the direction of reaming would be divergent from this line and angled toward the greater sciatic notch, as opposed to the center of the ilium.
Soft tissue landmarks, such as the tranverse acetabular ligament (TAL), have been used as tools for aiding in acetabular cup positioning.41 The TAL defines the line from the posteroinferior to the anteroinferior acetabulum. Placing the reamer parallel to the TAL represents the patient’s native anteversion, allowing the anteversion to be reproduced.
The height and depth of the component positioning have also been described relative to the TAL, with the component optimally sitting just underneath the TAL, as does the native acetabulum. If the component is sitting superior to the TAL, the cup position (and hip rotation center) has been moved superiorly, which can affect leg length as well as the hip rotation center. If the component is sitting deep to the TAL, the cup position has been medialized, and the offset has thus not been reproduced. An offset liner can then be used to restore the offset, such that the liner sits almost flush with the caudal end of the TAL.
Archibold et al documented 1,000 cases in which the TAL was used as the only guide to acetabular cup positioning. The ligament was identified in 99.7% of cases, and the incidence of dislocation in the first 8 months postoperative was 0.6%, thus claiming success with this technique even in patients who had a minimally invasive surgical (MIS) approach, defined as a posterolateral incision of 10 cm or less.41
The femoral component concerns include stem version, stem offset, and neck length. The landmarks used for the femoral component are various aspects of the proximal femoral anatomy, which can be identified radiographically in most cases but also should be identified intraoperatively.
Rotational alignment, or version, of the stem to the appropriate femoral anteversion influences allowable hip motion before impingement occurs and also affects abductor tension.4 Smaller femoral retroversion can be associated with early dislocation as well.46
Femoral offset is measured from the center of the head to the greater trochanter. Reconstruction of the femoral offset restores the biomechanics of the hip, specifically the abductor lever arm. Proper restoration of offset improves hip motion and reduces the risk of dislocation47.
Femoral neck length is measured from the center of the head to the lesser trochanter. Special attention should be paid to the relationship between the head center and the top of the greater trochanter. Attention should be paid to all femoral component factors to effectively yield leg length equality along with overall stability.3
LLD after THA - What to Do?
An organized approach to the evaluation of patients with true or functional limb length discrepancy is imperative in treating this problem successfully, and equally as important for avoiding the mistake of operating for the wrong reason. Non-correctable causes of LLD, temporary initial functional LLD, and component placement issues need to be identified before treatment begins.
Non-correctable causes of limb length discrepancy -- such as spine deformity, arthritis of the opposite hip, shortening of the contralateral femur or tibia, and preoperative lengthening on the operated side -- need to be identified before considering revision surgery. The patient should be made well aware of the possibility of an LLD prior to the THA to avoid ticklish conversations after the surgery. The patient’s perception can be that the surgeon has done something wrong, when, in fact, from a technical perspective, the THA may have been done well. Oftentimes, the only thing the surgeon has not done well is to discuss the possibility of LLD the patient preoperatively.7
Post-operative functional LLD is a phenomenon in which surgery creates a temporary tightness of the hip soft tissue structures, resulting in a pelvic obliquity (tilting towards the operated side) and subsequently a perceived LLD. This can be appreciated by close inspection of the radiographs. While the legs may actually be equal, the patient feels a difference. With time, the contracted soft tissue structures can be expected to elongate to a more normal anatomic length. In general, patients can be counseled that most functional LLD resolves.20
Component placement issues cause direct and indirect leg lengthening. Direct leg lengthening results in issues related to cup placement or stem placement. Component positions that directly lengthen the limb include placing the acetabular component inferior to the teardrop or the femoral component with the center of the head substantially proximal to the tip of the greater trochanter.
Indirect leg lengthening is caused by issues related to cup orientation, stem orientation, and stem offset. Leg lengthening here occurs because of a need for the surgeon to obtain hip stability intraoperatively. A retroverted (or excessively anteverted) acetabular component can cause intraoperative instability for which the surgeon tends to use increased neck lengths or offsets in order to maximize soft tissue restraint and achieve stability. The same can be true for the femoral component. Failure to reproduce anatomic offset can lead to leg lengthening because of a need to tighten soft tissues to maintain hip stability7.
Component position issues should be evaluated through careful examination of the radiographs. Computed tomography (CT) scans should be ordered if component orientation is felt to be a possible cause of the LLD. When component malposition causes either direct or indirect LLD and the patient remains symptomatic despite non-operative treatment, appropriate revision surgery to correct the causative agents has been shown to reliably yield positive results.3
It must be stressed to the patient before revision surgery that hip stability is of paramount importance, and attempts at “shortening” the long extremity should not compromise this fact. The surgeon must be prepared to revise both the acetabular and femoral components and have the option of constrained liners available if decreased limb length, despite appropriate positioning, results in soft tissue laxity and instability. No guarantee of equality should be given to the patient, and the risk of instability should also be explained.
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