Digital technology is rapidly making its way into hip replacement surgery. CT is used to analyze the patient’s bones, surgery is planned on a computer, and robotics and navigation are used to confirm position and changes during the operation. These advances aim to restore each patient’s unique joint with greater precision.
However, measuring accurately and actually achieving the measured target are two different matters. Even if the required leg length and horizontal distance of the joint are known, the plan may be hard to carry out if the prosthesis being used cannot offer that combination. This is why implant design and product range deserve attention alongside the advance of digital surgery.
■ Leg length, offset and stability: the three things hip surgery must restore
The key elements to restore in hip replacement surgery are leg length, offset and stability. If leg length is the vertical dimension, offset is easiest to think of as the horizontal distance. Femoral offset is the distance from the center of the femoral head to the femoral shaft axis and relates to the lever through which the surrounding muscles exert force. Even when length is correct, an inappropriate offset can affect muscle efficiency and joint stability.
The component of the hip implant inserted into the femur (thigh bone) is called the femoral stem. The stem must be fixed securely in the bone, and at the same time it must deliver the length and offset the patient needs. Selecting a stem that fits the bone and restoring the patient's joint anatomy must be considered together.
■ The stem that fits the bone and the offset the patient needs may differ
Many cementless stems are fixed by fitting them tightly into the space inside the bone, known as press-fit fixation. The width and shape of the medullary canal therefore influence the stem body size and insertion position. Although it varies by product, length and offset often change together as the stem body size increases.
The problem is that the width of the canal and the offset a patient needs are not always proportional. Some patients have a narrow canal but need a large offset, while others have a wide canal but need a small one. A small stem may fit the bone well but provide too little offset, and a larger stem may achieve fixation but make the offset larger than necessary.
Changing the head length does not solve every problem. Because the head attaches to the angled neck of the stem, a longer head increases both length and offset. An attempt to make up horizontal distance can end up lengthening the leg as well. The fact that “a size that fits the bone was chosen” is therefore not enough to conclude that length and offset are also appropriate.
■A design that lets stem body size and offset be chosen separately
This is where the range of available Exeter stem sizes becomes particularly relevant. Exeter is commonly known as a cemented stem, but another important feature is that it offers several body sizes for each offset. For example, different body sizes can be selected within a 44 mm offset. Not every size is available for every offset, but the range allows body size and offset to be treated as separate selection criteria.
In addition, leg length can be matched by adjusting insertion depth within a range that ensures an appropriate cement mantle thickness and alignment. After assessing joint motion and stability with trial components, surgeons can also adjust stem anteversion-the forward rotational orientation of the stem neck-before final fixation. These features should be understood not only in terms of the fixation method but also from the perspective of how the joint is restored.
■ Expanding the options is a task for surgeons and manufacturers together
Designs that change offset while keeping length constant are already in use among cementless stems. What matters is whether these options sufficiently accommodate differences between individual patients. Widening the range so that a larger offset can be chosen for a smaller body, and a smaller offset for a larger body, could help patients who were difficult to treat with existing sizes. However, because cementless stems are constrained by the insertion position at which they achieve stable fixation, adding more offset options alone will not solve every leg length adjustment issue.
When expanding options, the strength, initial fixation and long-term safety of each combination must be verified. Yet the need for verification is not a reason to stop development; it is a task to be solved during development. Surgeons should also document cases in which existing implants cannot adequately meet patients' anatomical restoration goals and share these findings with manufacturers. Manufacturers, in turn, should analyze those needs and respond with designs that accommodate patient differences more broadly.
■FAQ
Q1. Does planning surgery with digital technology automatically make hip replacement more accurate?
A1. No. Even if CT, robotics and navigation measure the target precisely, the plan is hard to achieve if the prosthesis cannot provide the required combination of leg length and offset. Implant design and product range must be considered as well.
Q2. Why might the offset be wrong even when the stem fits the bone?
A2. With many cementless stems, length and offset change together as the stem body size increases. But canal width and required offset are not proportional, so a small stem may give too little offset and a large stem too much.
Q3. Can changing the head length fix an offset problem?
A3. Not entirely. The head attaches to the angled neck of the stem, so a longer head increases both length and offset. Trying to make up horizontal distance can also lengthen the leg.
Q4. What makes the Exeter Stem different?
A4. Although known as a cemented stem, it offers several body sizes for each offset, so body size and offset can be chosen separately. Leg length can be matched through insertion depth, and anteversion can be corrected before final fixation.
