Methods for estimating leg length discrepancy:
Emphasis to the planning
and performance of total hip arthroplasty

A.X. PAPADOPOULOS[1], I. TSOTA[2], P. MEGAS[1]
[1]Orthopaedic clinic, [2]Department of Radiology, Patras University Hospital

Mailing address:
A.X. Papadopoulos
Papanikolaou St 1, Rio - Patras 26500
Tel: 0610-999555, 0610-420496, Fax: 0610-994579
E-mail: paprod@hotmail.com

ABSTRACT
Leg length inequality or anisomelia as a result of congenital or acquired conditions is an existing and not rare problem in orthopedics and traumatiology. A large percentage of this problem is associated with the hip region and is revealed during the planning and performance of total hip arthroplasty. The purpose of this study is to report the methods which have been developed for the estimation of leg length inequality and which include preoperative clinical measurements, imaging studies using radiographs, ultrasonograms and digital tomographies and to record the preoperative and intraoperative methods used for leg length estimation with the application of specific counters during planning and performance of total hip arthroplasty.
keywords: leg legth inequality or anisomelia, congenital disease of the hip, arthoplasty in congenital dislocation of hip.

INTRODUCTION
One of the fundamental problems of the contemporary lower limb orthopedic surgery and specifically of the hip region is the determination of the proper leg length preoperatively as well as intraoperatively. For several orthopedic conditions both congenital and acquired it is necessary to be achieved an easy and harmonious gait, following surgery which is ensured by the proper and functional length of the lower limbs. Postoperative leg length difference of more than 1cm may cause serious difficulties to the patient, which may require the use of lift shoes to compensate for them.
Two parameters have to be considered in the effort to determine the ideal length of the limbs. The first is associated with the real anatomic length of each limb and can be measured between specific anatomic landmarks. It is affected by congenital or acquired abnormalities in any anatomic structure located between the femoral head and the heel. The second parameter refers to the functional length of the limb, which allows a neutral pelvic position in the standing posture and walking without lameness. Functional leg length can be affected by disorders which are not associated with anatomic anisoscelias (scoliosis, deficiency of the abductor muscles, contractures of the hip or knee e.tc.)[9,28]. Also in some cases small anisoscelias may not affect the functional length and normal walking. Classification of the causes which lead to anisoscelias are listed in table 1.
Regarding osteoarthritis and hip dysplasias, the patient must have following total hip arthroplasty, besides a painless and functional joint, a totally functional limb, so as to walk without lameness or crutches. Failure of leg length equalization following total hip arthroplasty with a consequence a shortening of the limb has been guilty of significant aseptic loosening of the materials and can lead to an unstable hip. On the other hand, an extreme lengthening may cause sciatic nerve palsy and secondary osteoarthritis of the contralateral hip. It is made clear that the leg length inequality is associated with various conditions such as congenital hemimelias, shortening due to previous trauma and spine disorders. A significant problem is that of postoperative lumbar pain due to unequal distribution of the weight, pelvis tilting towards the shorter limb and consequent spine scoliosis. A technically correct arthroplasty may turn into failure due to anisoscelias consequences[7].
Especially in cases of congenital hip disease, where the new acetabulum is in lower position regarding the pseudo-acetabulum the problem is a major one, since many times the leg length on the dislocated side is already preoperatively longer. In such cases correct planning of the position of the new acetabulum, correct determination of the neck osteotomy level but also a shaft shortening osteotomy are all necessary in order to restore leg length inequality and avoid problems of a possible extreme tension of the soft tissues about the hip such as muscles, vessels and nerves owing to overlengthening of the limb.

A review of the literature reveals that the problem of leg length discrepancy following total hip arthroplasties does exist, despite the available methods of determining the desirable length. Leg length inequality varies in average from 2,8mm to 16mm, while it can often reach 30mm. It is characteristically mentioned by Love and Wright[19] that following unilateral total hip arthroplasty, up to 18% of the patients presented with a 1.5 cm limb overlengthening compared to the ipsilateral limb, while 6% of them considered this problem serious and required shoe correction for leg length equalization.
Williamson and Reckling[32] consider that following total hip arthroplasty a consequent leg inequality is usual. The average leg length inequality in their series was 16mm, while 27% of their patients required a contra lateral shoe lift for correction.
Djerf and Wahlstrom[6 ]describe leg inequalities up to 50% in a follow up study of total arthroplasties. An overall estimation of the literature determines that in 50% of total hip arthroplasty cases leg inequality of more than 1cm occurs, while 15-20% of the patients require shoe correction. The necessity to confront this existing problem has led to the development of many different methods for leg length determination both at the examination bed and on the operating table. A review of the literature led us to sum up the main ones.

 

Figure 1. Angle A, which is formed between the tangent of the top of the sacrum and the perpendicular line, corresponds to the functional leg length discrepancy between the two lower extremities.

A. TECHNIQUES OF MEASURING THE LOWER EXTREMITY LENGTH PRE-OPERATIVELY
(Table 2)
1) CLINICAL METHODS
a) Placement of certain height base
It is a well-known old method in which lift blocks of a certain height are placed under the shorter limb. During the measurement the feet are levelised, while the knees and the hips are in complete extension or, if this is not possible because of contractures at the same degree of flexion. The purpose is the levelisation of the pelvis, which assures a Ôcomfort' leg length for the patient. The advantage of this method is that the measurement is focused in the leg functionality and not in the strict limb length. This way, even in cases of gait anomalies because of spine disorders (e.g. scoliosis) the ideal leg length for an easy moving can be determined. A disadvantage is the easy impairment of the measurement because of hip abduction, adduction or flexion.
b) Measurement between two landmarks use of measuring tape
The patient either stands up in the standing posture, or lies down with the hips and thighs at full extension and the body in neutral position. The measurement refers only to the anatomical length and is reliable in known conditions (congenital or acquired) which affect it. We can measure:
i)The distance between umbilicus-medial malleolus, or between xyphoid recess-medial malleolus
Pelvic obliquity results in a misleading leg length discrepancy, because the measurement is made between an non-fixed landmark (umbilicus) and a fixed bony landmark (medial malleolus).
Furthermore the method is amenable to errors in cases of limb contractures and hip abduction-adduction.
ii) The distance between anterior superior iliac spine knee - medial malleolus
The measurement is made between two fixed bony landmarks and is thus more accurate. In spite that, an error may occur due to contractures, obesity, or previous surgical procedure which may have altered the normal structures.

Figure 2. MŸller overlay template.

2) IMAGING TECHNIQUES
A) RADIOLOGIC TECHNIQUES
For these techniques the pelvis has to be in an horizontal position, which is confirmed by observation or with radiologic evaluation by imaging the iliac crests, the sacrum and the lumbar spine. The angle which is formed by the tangent from the iliac crests or the tangent from the superior portion of the sacrum with the perpendicular axis refers to the functional length difference between the two limbs[13] (figure 1).
a) Measuring the distance between the acetabulum and the lesser trochanter on the anteroposterior pelvic view
This method is of valid in cases of leg length discrepancy owing to hip anomalies (dysplasia, osteochondritis) and is used for the determination of the osteotomy level. The measurement is made on an anteroposterior view of the pelvis. The pelvis is level with the femoral and tibial condyles parallel to each other and the central beam is perpendicular towards the symphisis pubis[12]. Measurement of the distance between the acetabulum and the tear drop is even more reliable, because it is not dependent on minor pelvic rotation, as the tear drop is more eccentrically located.
b) Measuring the distance between the lesser trochanter and the ischial tuberosity on the anteroposterior pelvic view
That way are measured differences attributed to anomalies located above the trochanter level in the hip region. On the true face pelvic view there are three parallel lines perpendicular to the long axis of the body, running the first one tangentially to the ischial tuberosity and the other two from each lesser trochanter. Thus by measuring the difference in the distance from the lesser trochanters of each femur to the ischial tuberosity we estimate the length difference between the two legs and the required correction which has to be done during the arthroplasty[32]. Using this method we cannot estimate differences which are due to other but hip disorders. Furthermore in severe hip contractures face views are not reliable and consequently whatever measurements are derived from it.
c) Preoperative planning with overlay templates
This method was described by Müller[23], who used templates on the radiographs of a patient with osteoarthritis in order to determine measurements for the proper femoral neck osteotomy site (figure 2). The acetabular template is placed at the appropriate depth (immediately below the subchondral bone) in a 40° abduction and a 20° anteversion and the femoral template is placed in a total relationship with the lining of the medullary canal.
The hypothesis is based on the fact, that if the same amount of femoral head, and neck, intra-articular space and subchondral bone of the acetabulum is removed, is replaced with prosthetic implants of the same height, the leg length should remain equal. In cases of preoperative anisoscelia (estimated by other means) the correction is also estimated. The lesser trochanter has been used as a reference point for the measurements for the correct osteotomy site.
In more recent studies Woolson35 uses as an anatomic landmark to determine measurements for the correct osteotomy site, the superior aspect of the femoral head, which is better visualized and accessible than the lesser trochanter.
The ideal equation is:
Femoral head and part of the removed neck(X) + intra-articular space(~2mm) + removedsubchondral acetabular bone (~1mm) = thickness of acetabular prosthetic implant (A) + femoral head and neck prosthetic implant (B)
When A and B are known we can easily calculate the X.
An important aspect of this technique is that the horizontal axis of the new acetabulum has to remain the same with the old one. An error regarding few mm may be encountered due to excessive superior acetabular reaming. The overlay templates are enlarged by 20% in order to match the average magnification of bone images on plain radiographs. This method cannot be applied in cases of congenital hip dislocations. It may also lead to errors in cases of contractures. It is ideal for the average patient with osteoarthritis who suffers from a mild leg length discrepancy and has normal anatomy[8,15].
d) Orthoradiography
The measurement is made using one to three radiographic views (depending on leg length), which include the lower limbs from the top of the iliac crest to the ankle (intermediate measurements can also be made e.g. from the lesser trochanter to the medial knee jointline)[18]. Measurement errors are due to central beam divergence, as well as to limb contractures, where a three dimensional deformity is presented on a single level.
e) Ultrasonography
Real time ultrasonography has been used as an alternative method for determining leg length[30]. The patient is examined in the erect position with the hips and knees straight and with equal weight on both legs. A 5 MHz transducer is used which is fixed horizontally in a holding device on a rack. The rack serves for holding and moving the transducer and has a millimeter scale. The transducer is moved vertically up and down with a cranked handle and the level is read on the millimeter scale. Scanning is performed from the anterolateral aspect of the proximal femur. The transducer is moved vertically to locate the most proximal part of the femoral head, which represents the leg length. The length of the lower legs can be determined separately by measuring the level of the proximal margin of the tibial condyles. Leg length measurements using ultrasound can be also performed with the patient in the supine position, providing that certain system adjustments are made. The ultrasound measurements provide relatively reliable results but the method should be carried out by experienced orthopedic surgeons during the clinical evaluation. Its main disadvantages are related with the cases of contractures where the leg axis differs from the rack axis and when the proximal femur is abnormal and the top of the femoral head may be difficult to determine.
f) Computed tomography (CT) evaluation
The anatomical length of the entire lower extremity can be accurately assessed on the initial 2 dimensional (2D) CT topogram (sensitivity <1mm)[2,10,11] (figure 3). The 3D image which is obtained from digital reconstruction of the scans can provide useful information regarding accurate spatial measurements of the entire limb or part of it. Contractures, which may lead to wrong estimations using the classical radiographic measurements, can be accurately estimated with 3D imaging. In the recent years CT has been used for the preoperative planning of a total hip arthroplasty as it can provide important three-dimensional anatomic details especially in cases of the dysplastic hip[36]. So the true difference in the length of the lower extremities can be estimated as well as the osteotomy level in order to length equalization to be achieved. The difference in the length of the lower extremities can be obtained from measurements on the topograms. Also the exact site of the osteotomy can be determined before surgery based on the leg length discrepancy and the anatomic position of the true acetabulum in order to restore leg length inequality.
Computed tomography scans are used for three dimensional reconstructions, which provide important anatomic details regarding the entire lower limb, not obtainable with conventional radiographs. A method for better preoperative assessment and selection of the optimum femoral implant was established by Xenakis et al[37] using a CAD system. The implant was selected by comparing the geometry of the femoral implant prior to implantation with the geometry of the medullary canal of the proximal femur. CT and CAD also provide details about the morphology and dimensions of the true acetabulum and the bone stock and assist before surgery in determining the need the exact location and the degree of a rotational osteotomy. That usually applies in cases when the leg length of the dislocated side is longer than the contralateral leg. Shortening osteotomy of the dislocated femur prevents tightening of the surrounding muscles, vessels and sciatic nerve.

3 4
Figure 3. Estimation of the anatomic femoral length using the CT topogram.
Figure 4. Intraoperative hip length calculator.

B) TECHNIQUES OF MEASURING THE LOWER LIMB LENGTH INTRAOPERATIVE (table 3)
1) Subjective estimation
The classical technique for measuring leg length during total hip arthroplasty includes comparison of the level of the knees with the level of the heels. This assessment of leg length is unreliable as it depends on surgeon's subjected estimation and uses no metric system. Especially when the operation is performed with the patient in a lateral position, the error is even greater because the two limbs are compared in flexion at the joints. The lateral position is different from the neutral position where the measurements are taken and many times the deviations are considerable.
2) Shuck test
The stability and mobility of the hip joint following prosthetic implantation may be assessed during surgery by the so-called ÔShuck test', which is based on the tension of the soft tissues surrounding the hip joint. By evaluating soft tissue tension some surgeons indirectly estimate leg length. The assessment of "Shuck test" includes applying traction to the lower extremity and determination of the amount of distraction of the total hip components.
If the prosthetic femoral head distracted more than 0.5 cm from the socket, the limb was assessed as being too short and a head with a longer neck was then tried. If distraction was not possible, the limb was assessed as being too long and the femur was reamed further in the distal direction. Alternatively, a prosthetic head with a shorter neck was used, and the hip biomechanics were assessed again at this stage. This technique provides a quick and not detailed leg length estimation as it is influenced by muscle laxation due to general anesthesia, the force applied to distract the joint and the amount of soft tissues release. It can also lead to limb overlengthening as the surgeon uses a longer neck in his effort to produce soft tissue tension in order to make the joint more stable.

3) Metric systems
a) Measurement between fixed landmarks around the hip joint
Such calculators have been used during total arthroplasty in a limited range, often with very good results[3,4,14,16,17,20,22,23,33,34]. In all cases the initial measurement is made before dislocating the femoral head, with the hip joint at the neutral position. Fixed anatomic landmarks are chosen on both sides of the hip, located on the iliac bone (either on the iliac spine, or the iliac wing 2-3 cm above the acetabular brink) and on the femur (usually on the major trochanter) (figure 4). Pins, or screws are placed on those landmarks and metric systems are adjusted over them. If the limb is shorter, a head with longer neck is tested, while if it is longer, the prosthesis is threaded deeper and a shorter neck is placed. The results of this method from different series have been satisfactory, but the method is not widely applied. Those systems which provide better support at the attachment points and those which are supported around the hip in three points (two at the iliac and one at the femur)[14,34] seem to be more reliable because they define more properly the measurement level. Emphasis should be pointed to the hip position during measurement as abduction, or adduction may alter the results. For insuring proper neutral patient position, when the patient is at the lateral position the use of "carpenter level" (water bubble) has been suggested.
b) Measurement of the resected femoral bone
The measurement is made after femoral head dislocation and prior to osteotomy. The caliper measures the distance between the highest bony point of the femoral head and the chosen osteotomy level. (This location can be preoperatively determined with the use of radiographic templates as it is aforementioned). After that when the femoral component has been placed, it is threaded inside the medullary canal to that point as the length of femoral head and neck left behind will be equal to that of the initial measurement. The caliper was designed by the Belfast Hip Group in Musgrave Park Hospital and was applied in 796 patients. In the 94% of the patients the femoral component was placed with a length difference of up to 6mm, compared to that preoperatively estimated[25].

Figure 5. Total hip arthroplasty without cement, in congenital hip dislocation. A shortening osteotomy of 5cm at the femoral shaft below arthroplasty was deemed necessary for leg length equalization, following placement of the acetabulum and going down of the hip at its anatomic position.


Figure 6. Rotation center of femoral head and offset of femoral neck.

CONCLUSIONS
The great significance of the correct determination of the ideal limb length is obvious from the study of the movement and the functionality in the erect position and walking. Acquired conditions which lead to leg inequality may affect in a great degree the harmony of the movement even if this leg inequality is about 1cm in average. At the same time, disorders of the skeleton formation, which many times present large leg inequalities, demand thorough study, in order to lead, through modern lengthening techniques, to functional lower limbs. Understanding of the conceptions of anatomic and functional length may lead towards the right direction every interventional procedure in the hip area, operative or not.
Leg length discrepancy following total arthroplasty may be considered by a lot of surgeons as a complication of minor significance, nevertheless it can undoubtedly decide the result of functional recovery after such an operation. Most patients pay more attention to the subsequent equalization, especially those who have minor differences preoperatively and evaluate the arthroplasty result from their ability to walk without pain or helpers.
Careful planning and application of intraoperative measurement systems seem to decrease the percentage of severe leg length inequalities. Jasty and Harris[15] report that using such techniques, only 13% of their patients presented with leg length inequality following total arthroplasty, with a greatest deviation of 1cm. Planning of shortening or lengthening during total hip arthroplasty is of great importance in cases of preexisting inequality. Lengthening or shortening is mainly performed at the femur, since both the level of the osteotomy and the length of the femoral head may be modified. Specific problems occur in those cases that there is a preoperative limb shortening of more than 2cm and we desire to achieve great amount of postoperative lengthening. Such problems are encountered in congenital hip dislocation posttraumatic osteoarthritis, previous septic arthritis and ischemic necrosis of the femoral head. If the shortening has occurred in childhood, then overlengthening (<2cm) may lead to sciatic nerve palsy. Thus a thorough planning is required to avoid unpleasant consequences. Intraoperatively and in case where the limb has been lengthened for more than 2cm, we can palpate the sciatic nerve and evaluate the range of tension under which it is. If shortening of more than 2cm is desired, a trochanteric osteotomy is necessary to ensure postoperative stability which is threatened by the laxity of the abductors.
Gait analysis following total hip arthroplasty show that the functional leg length and the harmonious gait are achieved, when besides the anatomic length, the natural geometry of the hip region are ensured[27,29]. The location rotation center of the hip is of great importance. This position is determined by the distance between the center of the femoral head and the lesser trochanter. It has been proven that removing or lengthening up to 1cm of the rotation center do not actually seem to affect the movement, while, on the contrary its shortening (in relation to the lesser trochanter) leads to decrease of the movements both in the hip and the knee. Such a situation probably occurs because of the decrease of tension and the power of the hip muscles. A similar part is played by the so-called "offset" which corresponds to the vertical distance between the center of the femoral head and the central axis of the medullary canal (figure 6).
From the evaluation of the previously mentioned methods of length determination, it seems that a combination of more than one is needed to fully estimate the length issue. A review of the literature regarding especially the area of the arthroplasties, shows that a thorough clinical estimation combined with a good preoperative planning supplemented with the application of an intraoperative caliper system seems to be the most reliable way of approaching the problem.

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