Summary of this page: Total hip arthoplasty is meant to relieve pain, allow the patient to walk with a more normal gait, and result in a highly satisfied patient. One of the most troublesome problems after total hip arthoplasty for both patients and orthopedic surgeons is when the patient has unequal leg lengths after surgery, which is referred to as a “leg-length discrepancy”. While there are a few immediate complications that can occur as the result of a leg-length discrepancy, such as a stretch injury to the sciatic nerve or peroneal nerve, back pain as the result of an uneven pelvis, or an abnormal gait, the most frequent outcome is a dissatisfied patient and an unhappy surgeon. While careful preoperative planning and intra-operative techniques can decrease the chance of the patient noticing a leg-length discrepancy after the operation, a good course of action is to have the patient and the surgeon agree in advance on what constitutes equal leg lengths, define a standard for evaluating leg lengths, and then using a standardized X-ray technique for measuring leg lengths. This ensures that every one is on the same page when it comes to this issue. In our experience, the anterior approach has a few benefits over the posterior approach when it comes to making sure the leg lengths are equal after surgery. This is primarily because the patient is flat on their back during surgery, which makes it much easier to control the alignment of the pelvis, and also because a C-arm — an intraoperative X-ray machine — is used in anterior hip surgery so that an objective measurement of leg lengths can be made before the procedure is finished.
Recent data produced by a oversight agency called the Joint Commission on Accreditation of Healthcare Organizations (JCAHO) listed 6 major “events” that are relevant to orthopedics out of 19 frequent adverse medical events that deserve particular attention. These “events” are occurrences that can be seen as medical errors because they may be preventable in some instances, and in this study, leg length inequality accounted for 4.7% of the total number of medical errors investigated by JCAHO. Other events, such as patient falls while in the hospital, are relevant to orthopedic surgeons, but their prevention is a bit more complex.
The most frequent error in limb lengths after total hip replacement is that the operated hip is longer than the native hip. Sometimes this may be necessary in order to achieve stability of the replaced joint. When the patient is being operated on for arthritis complicated by congenital dislocation of the hip for example, it may be necessary to lengthen the leg in order to return the hip center to its normal location. When the leg is lengthened, it may place increased stress on the sciatic nerve, stretching the nerve, and this can produce what we call a nerve palsy. This will typically manifest as burning pain down the course of the sciatic nerve, numbness in the lower leg or foot, and sometimes muscle weakness in the foot and leg. If patients are left with uneven leg lengths after surgery and their gait is abnormal, they can develop low back pain or other gait abnormalities that will cause them discomfort. However, most of the time, the most common complaint from a patient with a leg length discrepancy is just that they are dissatisfied. Indeed, dissatisfaction with unequally leg lengths after total hip arthroplasty is the SINGLE MOST COMMON reason for litigation against orthopedic surgeons. Lets figure out a way to avoid that from happening.
Over the years, the surgeons who choose to perform total hip arthroplasty through a posterior approach have developed a number of ways to either directly or indirectly measure leg lengths before the incision is sewn up. These include using careful pre-operative planning with overlays that anticipate what changes in leg lengths are going to occur during the course of the joint replacement, placing markers on the pelvis and the femur so that the distance between the two objects can be measured before and after the joint is replaced, taking an intra-operative X-ray to confirm the limb lengths, and feeling the length of the legs through the drapes. In my experience with these techniques, they are not always 100% foolproof. Pre-operative templating is certainly helpful, but minor variations in the bone and the local anatomy can result in subtle changes in the way that the implants fit into the patient. The use of pins that are tapped into the femur and the pelvis as a standard of measurement is not 100% secure because they can move during surgery. Taking an X-ray with a patient lying on their side (which is the standard position for posterior total hip arthroplasty) can be very difficult, especially if it is a large patient and their position has changed on the table during the operation. This frequently happens because once a patient is covered with all of the barriers that we use to create at sterile field — the process of “prepping and draping” — it can be very difficult to make sure that the patient is correctly aligned when the X-ray is taken. If the patient is not perfectly square with respect to the table then the X-ray technician will not be able to get an accurate picture of the pelvis, and using the the intra-operative X-ray to assess leg lengths can be problematic. While it is desirable to aim for a “bulls-eye” and restore the legs to exactly equal lengths, this is not always perfectly possible. There is some debate about what constitutes an acceptable leg length discrepancy after surgery, with some surgeons advocating for anything less than 1cm (less than 1/2 an inch) as a very acceptable threshold. It is pretty difficult to identify any problems that are realistically caused by a difference in leg lengths of less than 1 centimeter, since a large proportion of the general population has a leg length discrepancy of about 1/2 of an inch by virtue of the natural underlying asymmetry of the human body — before the ever develop arthritis or have a joint replacement.
In my hands, one of the major advantages of the direct anterior total hip arthroplasty technique is what I perceive to be an increased ability to determine leg lengths accurately. The patient is lying flat on their back, so even if they are very large, the pelvis is still perfectly square to the table, and therefore perfectly square to the X-ray beam. Also, the intra-operative C-arm can be used to make sure that the femoral and acetabular components are in the correct place, and in general, it is much easier (and represents less of a risk to the sterility of the procedure) to take multiple X-rays with the C-arm than it is do the same thing with a portable X-ray unit, which is what is required during most conventional setups for posterior total hip arthroplasty.
Here is an X-ray of the pelvis of someone who is going to have their right hip replaced. A horizontal line has been drawn across the bottom of the pelvis, just touching a part of the pelvis called the ischial tuberosities. Notice that the pelvis is perfectly “square” with respect to the viewer, and there is a high degree of symmetry in the anatomic landmarks from one side to the other. The distance between the horizontal line and the top of the bump of bone on the inside of the femur (the lesser trochanter) is the most commonly used method for determining leg lengths. This is because the horizontal line is a PELVIC reference, and the top of the lesser trochanter is a FEMORAL reference. Notice that on this X-ray, the top of the lesser trochanter on the patient’s right side (this x-ray is oriented as if the patient is looking at us) is displaced superiorly with respect to the left side. This is because the arthritic process has thinned the cartilage inside the hip joint, and the erosion of the cartilage has resulted in a progressive shortening of the limb. The goal of a total hip arthroplasty in this instance would be to replace the joint in such a way that the distance between the horizontal line and the top of the less trochanter is equal on both side.
Here is an X-ray of a patient without arthritis, and the relationship between the horizontal line and the top of the lesser trochanter is equal. I am trying to achieve the same relationship between the pelvis and the femur after a hip operation.
Here is a picture of a relatively large (but by no means very large) patient getting prepared to have a direct anterior total hip arthroplasty. Notice how the patient is flat on their back so that the pelvis is perfectly square. If this same patient were to lie on their side, it would be much more difficult to control the position of the pelvis because the “soft tissue envelope” around the patient makes it difficult to feel the landmarks of the pelvis and it also makes it difficult for the intra-operative positioners to hold onto the pelvis securely. Bottom line — it is harder to orient large patients correctly 100% of the time when they are on their side, especially all throughout the procedure.
Here is a picture of the same patient during the middle of the procedure. In this instance, the C-arm has been prepped and draped so that in can easily be pulled in and out of the surgical field whenever we need to verify the position of the components. For example, in the next picture, a spot view has been taken of the acetabular reamer after preparation of the socket side of the hip replacement. This will verify that the socket has been appropriately reamed to accept the new component.
In the second picture, the femoral stem has been inserted, and the C-arm is being used to verify that all of the components are in the correct position and that the relationship between the anatomic landmarks on the acetabular side and the femoral side of the joint replacement are all correct.