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.
Leg Length / Neck Length & Offset:
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).