How Your Hip Replacement Implant Is Chosen and Sized
Patients often assume a hip replacement comes in small, medium and large. It does not. Before you arrive, I plan your implant on a calibrated X-ray using templating software — the stem size, how far your femur should sit out from your pelvis, and what your leg length should be, worked out to within millimeters. Then I confirm it during surgery with live imaging rather than trusting the plan blindly.
Key takeaways
- Hip implants are not one-size-fits-all — anatomy varies enormously between patients.
- Templating is done before surgery on a calibrated X-ray, planning size and position to within millimeters.
- Three things are planned: stem size, offset (how far the femur sits out from the pelvis), and leg length.
- Offset sets the tension on the muscles around your hip — too little feels weak and unstable, too much feels tight.
- Heads and liners come in incremental sizes that fine-tune length and offset during surgery.
- Live X-ray imaging during the operation confirms cup position, offset and leg length before anything is final.
- You can send X-rays ahead of a visit and get useful preliminary templating without travelling.
The short answer
Patients often assume hip implants come in small, medium and large, and that the surgeon picks one in the moment. Your implant is planned before you arrive, sized to your own anatomy on a calibrated X-ray, to within millimeters.
That plan predicts three things — the size of the stem that goes into your femur, how far your femur should sit out from your pelvis, and what your leg length should be. During the operation I then confirm all three with live imaging rather than trusting the plan blindly.
Why one size cannot possibly fit
The range of human anatomy is far wider than most people imagine. I operate on professional athletes well over six feet tall, and on patients whose femoral canal is narrower than the smallest stem some implant systems offer.
And it is not only overall size. The shape differs from person to person:
- The angle at which the femoral neck rises from the shaft — steeper in some people, flatter in others
- How far the neck projects sideways, which changes where the hip’s center of rotation sits
- The depth and orientation of the socket in the pelvis
- The width and shape of the femoral canal, which determines how a stem fits and grips
Two patients of identical height can need meaningfully different implants. This is why implant systems come in wide size ranges with incremental heads and liners — not as marketing, but because the variation is genuinely that large.
What templating actually is
Templating is planning the operation on an image before performing it.
We take an X-ray that is calibrated — taken with a reference marker of known size in the field, so that measurements on the screen are true to scale rather than approximate. That image goes into planning software, where implant templates are overlaid on your own anatomy and sized until the fit is right.
You can start this from a distance
If you send X-rays ahead of a visit, we can do preliminary templating that is close to accurate. It makes a first appointment — or a second opinion from out of town — considerably more substantive than a conversation without images.
The three things I plan
Stem size
How large a stem your femoral canal will accept and hold. Too small and it will not grip properly; too large risks the bone. The plan gets this close before surgery, and the fit is verified as the stem goes in.
Offset
Offset is how far your femur sits out sideways from your pelvis. It sounds technical and it matters enormously, because it sets the tension on the muscles around your hip — particularly the abductors, the muscles that keep your pelvis level when you stand on one leg.
- Too little offset — the muscles are slack. The hip can feel weak, you may limp, and instability risk rises
- Too much offset — the soft tissues are over-tightened, which can feel tight or uncomfortable
- The goal is to restore the offset you already had, because your muscles are already built around it
Leg length
Patients worry about this more than almost anything else, and reasonably so. The plan measures the difference between your legs before surgery. Where a hip has genuinely shortened from arthritis and wear, we can restore some of that length. The aim is to match what you have — and it is measured deliberately rather than eyeballed.
The femoral side — stem and head
The stem provides the foundation, but the head — the ball that sits on top of it — is where fine adjustment happens.
Heads come in incremental sizes, each stepping up length and offset by a small amount. That means that once the stem is seated, I can still fine-tune both by choosing the head, without changing the stem. It is the difference between a hip that is approximately right and one that is right.
I use a ceramic head on highly cross-linked polyethylene for every patient — the reasoning is in hip replacement bearing surfaces.
The socket side — shell and liner
On the pelvic side there are two components: the shell that anchors into bone, and the liner that sits inside it and articulates against the head.
Most shells are press-fit — sized slightly larger than the socket prepared for them, so they wedge in and bone grows into the surface over the following weeks. Screws can be added where extra initial stability is needed.
The liner is where the options live:
- Standard liners for the majority of patients
- Offset or lateralised liners, which add a small amount of length and offset
- Lipped or face-changing liners, which add coverage in a chosen direction for extra protection against dislocation
- Dual mobility constructs, which add a second bearing surface and allow more motion before the hip can dislocate — useful for patients at higher instability risk
There is also version — the rotational orientation of the shell in the pelvis. Getting it right matters for stability, and a liner can be used to fine-tune the effective version once the shell is seated.
Why the system matters more than the brand
The cup system I use accommodates all of these — offset liners, face-changing liners, dual mobility — from the same shell. That means the decision can be made from what your anatomy needs on the day, rather than being constrained by what is compatible with the component already implanted.
Confirming it during surgery
A plan made on a screen is a hypothesis. What makes it reliable is checking it while there is still time to change it.
During the operation I use live X-ray imaging, with the picture on a screen in the room. Before anything is final I can see and measure:
- Cup position and orientation in the pelvis
- Offset — whether the femur is sitting where it should relative to the pelvis
- Leg length compared against the other side
- Stem position and fit within the canal
If something is a few millimeters off, it gets corrected then — by changing a head, a liner, or the seating of a component. This is what navigation means in my practice: measuring what you are doing while you do it. It is not the same as robotic surgery, which is a separate technology, and I have written about that comparison in robotic versus fluoroscopy-guided hip replacement.
Why this matters to you
Almost everything patients care about after a hip replacement traces back to these decisions.
- Whether your legs feel even
- Whether you limp, and whether the hip feels strong when you stand on it
- How stable the hip is, and whether you need precautions afterwards
- How natural the hip feels — the goal being a hip you stop thinking about
None of that is decided by which brand of implant is used. It is decided by whether the implant was sized and positioned for your particular anatomy — planned in advance and verified before you leave the operating room.
Frequently asked questions
How does a surgeon know what size hip implant you need?
Through a process called templating. A calibrated X-ray — one taken with a reference marker of known size so measurements are true to scale — is loaded into planning software, and the implant is sized against your own anatomy before surgery. That plan predicts stem size, offset, and leg length to within millimeters. It is then confirmed during the operation with live imaging rather than assumed to be correct.
What is offset in a hip replacement?
Offset is how far your femur sits out sideways from your pelvis. It determines the tension on the muscles around the hip — particularly the abductors that keep you level when you walk. Too little offset and the hip can feel weak and be more prone to dislocation; too much and it can feel tight. Restoring your natural offset is one of the main goals of the plan, and it is adjustable during surgery through the stem, the head, and the liner.
Will my legs be the same length after a hip replacement?
That is the goal, and it is planned for specifically. Templating measures the difference before surgery, and live imaging during the operation confirms it before anything is finalized. Where a leg has genuinely shortened from arthritis and wear, we can restore some of that length. Perfect equality is not always achievable, but it is measured deliberately rather than left to chance.
Are hip replacement implants one-size-fits-all?
Not remotely. Patients vary enormously — I operate on professional athletes well over six feet and on patients whose femoral canal is narrower than the smallest stem in some systems. Beyond overall size, the shape differs: the angle of the femoral neck, how far it projects, the depth and orientation of the socket. Implant systems come in wide size ranges with incremental heads and liners precisely because that variation is real.
Can you template my hip from an X-ray before I come in?
Yes, and it is genuinely useful. Send your X-rays in advance and we can do preliminary templating that is close to accurate, which makes a first visit — or a second opinion from a distance — far more substantive. The definitive plan uses a calibrated film taken with a reference marker, but a preliminary read tells us a great deal before you travel.
What is a dual mobility hip implant?
A construct with an additional bearing surface that allows more range of motion before the hip can dislocate. It is not needed for most patients, but it is a useful option for someone at higher risk of instability. The system I use accommodates it, which means the decision can be made based on what your anatomy and situation call for rather than being limited by the implant on the shelf.
Is computer navigation the same as robotic surgery?
No. Navigation means using imaging during the operation to measure and confirm what you are doing — in my case live X-ray, with cup position, offset and leg length displayed on screen before anything is final. A robot is a separate technology that physically assists in making cuts or positioning components. I have written separately about robotic versus fluoroscopy-guided hip replacement and why I prefer imaging guidance.
This article is for general education and is not a substitute for personalized medical advice. Recovery timelines vary by patient, procedure, medical history, and surgeon-specific protocol. Please consult Matthew Harb, M.D. about your specific condition.
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