What actually gets replaced

A total hip replacement does not replace the whole pelvis or the whole femur. In total hip arthroplasty, the damaged joint surfaces on both sides of the hip joint are replaced: the socket side in the pelvis, called the acetabulum, and the ball side at the top of the thigh bone, called the femoral head.

The usual hip replacement has four main parts: an acetabular cup placed into the pelvic socket, a liner that sits inside that cup, a femoral stem placed inside the thigh bone, and a new ball or head attached to the stem. Together, these parts recreate the hip’s ball-and-socket movement with a smoother artificial bearing surface. In plain terms, the cup goes in the pelvis and the stem goes down the femur; the operation replaces worn surfaces rather than the entire bones.

Hip hemiarthroplasty is different. In that operation, only the femoral head side is usually replaced, not the acetabular socket, which is why it should not be confused with a total hip replacement.

How the implant works day to day

Day to day, the implant works by creating a new low-friction meeting point inside the hip rather than restoring the exact joint a person had before arthritis. As the AAOS explains, the replacement ball moves against a liner inside the socket, forming a “smooth gliding surface”. That liner is the main bearing surface, and it may be made from plastic, ceramic or metal depending on the bearing combination chosen.

On the thigh-bone side, the femoral stem supports the new ball and transfers load into the femur during ordinary tasks such as walking, climbing stairs and rising from a chair. In AAOS terms, the goal of hip replacement is smoother movement, less pain and better function. It is still a mechanical reconstruction of the hip joint, not a brand-new natural hip, so feel and range can improve without being identical in every case.

Cemented, cementless, and hybrid fixation

Fixation describes how the parts of a hip arthroplasty are secured to bone. In Orthobullets terms, cemented fixation uses polymethylmethacrylate (PMMA) bone cement to create immediate mechanical hold at the implant–bone interface. Cementless fixation does not use cement; instead, the stem or cup is designed so bone can grow on to or into its surface over time, described as "ongrowth" or "ingrowth". These labels refer to the method of attachment, not to whether the bearing is plastic, ceramic or metal.

A hybrid hip replacement mixes the methods. NICE TA304 describes the common hybrid pattern as a cementless cup with a cemented stem. Fixation is also separate from surgical approach: a SPAIRE hip replacement, the muscle-sparing posterior approach associated with Prof Paul Lee, may still be cemented, cementless or hybrid. The balanced point is that all three strategies are established in modern hip replacement; the practical question is patient suitability, including bone quality and femoral shape, rather than a single winner for every case.

How surgeons match fixation to the patient

Matching fixation in hip arthroplasty usually starts with the femur rather than the birthday. Orthobullets highlights the Dorr classification as a practical guide on the femoral side: Dorr A and Dorr B femora, which generally have better shape and bone stock for press-fit stability, are commonly matched with uncemented stems, while Dorr C femora more often favour cemented stems. In plain terms, the surgeon is asking whether the bone can grip the implant reliably now and support biologic fixation over time.

Bone quality then becomes the next filter for patient suitability. Sources from Complete Orthopedics note that poorer bone quality, including osteoporosis or previous bone irradiation, may make cemented fixation more attractive in some hip replacement cases, whereas cementless stems are more often used in younger or healthier patients with good bone stock. Even so, age on its own is an incomplete rule. A 2025 study in patients aged 70 years and over reported no significant difference in 5- and 10-year stem survivorship or PROM improvement between cemented and cementless groups, which supports a more individual assessment.

The final decision is therefore a balanced comparison, not a universal cut-off. A 2020 PMC review argues that surgeon expertise, training, implant design and institutional practice all shape fixation choice, and NICE TA304 also reminds clinicians that construct cost differs, with cementless options often carrying higher list prices. In consultant-led planning for total hip replacement, including muscle-sparing posterior approach work such as SPAIRE hip replacement associated with Prof Paul Lee, fixation is still matched to the patient’s bone and overall circumstances rather than chosen by age alone.

How fixation differs from surgical approach

A patient example makes the distinction clearer than another label. In 2025, one person assessed for SPAIRE hip replacement under Professor Paul Lee’s muscle-sparing posterior approach may have a Dorr B femur and be planned for a cementless stem, while another person suitable for the same soft-tissue route may have Dorr C bone and receive a cemented stem. The approach has stayed the same; the fixation has changed because the bone and implant plan are different.

Seen that way, the two decisions answer different questions inside one hip arthroplasty. Approach choice concerns access to the hip joint and the handling of muscles and soft tissues; fixation choice concerns how the implant is secured once the joint is reached. A 2020 PMC review notes that surgeon expertise and patient factors both shape planning, so SPAIRE is one approach option in a balanced comparison, not a fixation method and not the right answer for every patient.

What to ask at your assessment

For a 2025 assessment, the most useful end point is a short question list that keeps the discussion on patient suitability and a balanced comparison rather than on labels alone.

• “Is this planned as a total hip replacement or a hemiarthroplasty, and which parts of the hip joint are being replaced?”
• “For the cup and the stem, is the fixation cemented, cementless or hybrid in the NICE sense, and what in my bone quality or femoral shape makes that the better match?”
• “If SPAIRE hip replacement is being considered, including the muscle-sparing posterior approach associated with Prof Paul Lee, what makes me a good or poor candidate for that approach?”
• “How will this fixation choice affect the early plan after surgery, including follow-up and any checks on implant stability?”

Hip Replacement Lincolnshire is part of the MSK Doctors group and accepts patients without referral. Book an assessment at hipreplacementlincolnshire.co.uk.

1. [1] Cementless versus cemented stems in patients aged 70 years or older undergoing total hip arthroplasty. (2025). https://doi.org/10.1016/j.arth.2025.02.008 https://doi.org/10.1016/j.arth.2025.02.008