The aim of hip replacement surgery is to give the patient a pain free hip joint which restores or maintains leg length and is not vulnerable to dislocation. In this article I will discuss how orthopaedic surgeons judge whether a replacement hip joint has been inserted correctly.

When a new hip socket is fitted inside the natural socket, the ball part of the joint is smaller than the native hip, which increases the risk of dislocation. Correct positioning of the socket can help reduce this risk. Think of the socket as an umbrella, where the handle of the opened umbrella represents the axis of the socket.

Orthopaedic surgeons use two terms to describe the orientation of the socket or its “handle”:

• Version
• Inclination

In Figure 1, it can be observed that the natural hip socket faces forward (anteversion), and the artificial replacement can be placed in normal or neutral anteversion, excessive anteversion, or retroversion/retroverted. A retroverted socket increases the risk of posterior dislocation when the leg is turned inwards, while excessive anteversion increases the risk of dislocation when the leg is turned outwards. The inclination of the socket refers to the degree to which the axis points downwards. The terms “open” and “closed” are often used, with an open socket being more vertically placed than its closed

counterpart. An overly vertical socket increases the risk of dislocation as the knee is brought towards the midline (adduction).

The acceptable range of these two measures are often referred to as the “safe zone,” which suggests that the risk of dislocation is reduced when the socket is placed within this zone. However, it is important to note that in clinical practice and clinical negligence, this concept is a simplification. Many hip replacements with seemingly poor socket positioning function normally, while some patients with apparently well-positioned components experience instability and dislocation. Hip joint stability is influenced by other factors, including the competence of the supporting soft tissues and the size and positioning of the femoral (thigh bone) component.

Similarly to the socket, the thigh bone component, which provides the ball of the artificial joint, is slightly pointed forwards (anterversion). Excessive anteversion increases the risk of dislocation when the leg is turned outwards, while retroversion increases the risk of dislocation during internal rotation. An anterverted femoral component may compensate for a retroverted acetabulum, and vice versa.

Lengthening the leg at the hip joint generally tightens the soft tissues and reduces the risk of dislocation. Restoring leg length is a necessary part of the surgery, especially since arthritis itself shortens the leg at the hip joint and restoring length aids long-term stability. However, excessive lengthening can result in noticeable limb length discrepancy after surgery, which is one of the common causes of clinical negligence claims. Shorter female patients are more sensitive to limb length discrepancy compared to taller male patients. The length of the implant’s neck (neck length) and the distance between the ball center and the thigh bone (offset) independently contribute to joint stability and also affect leg length (figure 2).

Surgeons strive to balance the risk of overlengthening to avoid dislocation with the risk of limb length discrepancy. This requires skill and experience, and intraoperative measurement of leg length is just one factor in achieving this balance. Assessing the tightness or looseness of the hip joint in an anaesthetized patient and predicting how the patient will feel when awake requires considerable skill and judgment. While limb length discrepancy is

often assumed to be the cause of postoperative dissatisfaction, increased offset (which is invisible by external examination) can also be a significant factor contributing to the subjective feeling that the hip is “not right.”

Assessing implant positioning requires radiographic image analysis, which may include the following:

• Scaled pre and postoperative pelvic radiographs
• Lateral hip radiographs
• Leg alignment (limb length) radiographs
• CT scan of the pelvis
• CT scan for rotational lower limb profile measurement (including both hips, knees, and ankles)

Detailed radiographic analysis using dedicated image viewing software and hardware in a diagnostic environment is necessary to determine breach of duty of care and causation in assessing implant positioning. It’s important to note that the commonly available DICOM medical imaging packages, often bundled with bespoke viewing programs for medico-legal purposes, may not be suitable for this purpose.

Editor Comment: “Inneg provide a bespoke viewer to our clients that allows the expert to review all imaging behind one easy-to-access GDPR-compliant share link. This is part of the complete record management package that we provide.”