It was written to assess the success of femoral head positioning of total hip replacements. The definition of the frames of reference and their calculations were developed by Prof. Bill Walters (Mater Hospital, Sydney) and Dr. Simon Harris (Imperial College, London).
You may click on the labels to the left of each of the options in the form above to obtain help with obtaining the position of the anatomical landmark. On each line of the form, enter the X,Y and Z components of the landmark position into the three boxes.
This hip offset calculator was developed by collaboration:
Simon J. Harris1
Assoc. Prof William L. Walter
Prof Justin Cobb1
*Disclaimer
By using the calculator, you accept this disclaimer in full. If you disagree with any part of this disclaimer, you must not use the calculator. Assoc. Prof. William L. Walter owns the intellectual property rights to the calculator. However, you may use the calculator for the stated purposes only. Whilst every effort has been made to ensure that the calculator is correct, its completeness or accuracy is not warranted and there is no commitment to ensure that the calculator is kept up to date.
To the maximum extent permitted by law, all representations, warranties and conditions relating to the calculator and the use of same (including, without limitation, any warranties implied by law in respect of satisfactory quality, fitness for purpose and/or the use of reasonable care and skill) are excluded. Assoc. Prof. William L Walter will not be liable for any costs, losses, expenses or damages (including whether direct or indirect, consequential, special, economic or financial including any loss of profits) whatsoever that may be incurred through the use of the calculator.
Nothing in this disclaimer will limit or exclude any liability however caused that is not permitted under Australian law or cannot be excluded under Australian law. This limitation and exclusion of liability governs all liabilities that are the subject matter of the disclaimer, including liabilities arising in contract, in tort (including negligence) and for breach of statutory duty.
The calculator is designed to provide a measure of femoral head offset from the femoral shaft in both the medial and anterior/posterior directions. It was written to assess the success of femoral head positioning of total hip replacements. The definition of the frames of reference and their calculations were developed by Prof. Bill Walters (Mater Hospital, Sydney) and Dr. Simon Harris (Imperial College, London)
While neck anteversion may be thought to give a measure of the femoral head relative to the femoral shaft, it does not give the whole story for two reasons:
For these reasons we have looked at an alternative method of describing the femoral head centre.
To obtain a measure of femoral head position independently of shaft insertion point and shaft tilt within the canal, we have defined two measurements, Anterior offset and medial offset. These measure the head centre relative to a specific frame of reference.
Some landmarks traditionally used to create frames of reference were deemed unsuitable for this work. The piriformis fossa is often used as an anatomical landmark, however the process of hip replacement will often remove this landmark rendering it unsuitable for use as a reference point. As we intend to measure the head relative to the shaft, the mechanical axis (head centre to knee centre) of the femur is also an inappropriate reference frame here.
We have chosen a proximal femoral shaft axis from just below (~20mm below) the lesser trochanter to approximately 120mm below the lesser trochanter as our reference axis.
This axis is created by approximating a circle to the shaft at each of these distances. A circle at each point is marked to best approximate the outline of the shaft and the centres of these circles form a line extended to the height of the femoral head.
To complete the definition of the frame of reference, the axial alignment of the axis is required. Currently we are using the trans-epicondylar axis to define a 'horizontal' (medial-lateral) reference for this axis.
This reference was used because it is already a commonly used reference. Other references such as the posterior of the condyles may be chosen in preference if required.
The measurement alignment process with the frame of reference works with the following order of rotations - all points are rotated with each rotation:
In addition to providing an axial orientation for the frame of reference, the trans-epicondylar axis also provides a measure of leg length, which should be compared with the contra-lateral leg for leg-length discrepancy measurements. The length measured is the vertical distance, in the frame of reference, from the femoral head centre to the trans-epicondylar axis mid-point.
For comparison purposes, if possible the hip replacement should be studied in conjunction with a natural contra-lateral hip, and the measurements compared between the two limbs
Positioning the femoral head centre is the same basic process whether the hip replacement (involved) or a natural (non-involved) hip is being measured. Starting with the natural head:
In this example, it is assumed that OsiriX is being used to make the measurements. Other CT measurement software is available, but the screen layout and features available may be different.
The first stage is to locate the cross-hairs at the centre of the femoral head. It will look circular in each view, but unless aligned correctly, the approximate circular outline in each view will have differing diameters. To ensure that a correct alignment is achieved, once positioned, outline the femoral head in each view with a circle and check that the diameter measured is equal for each.
Once accurately aligned, place the mouse cursor over the intersection between the cross-hairs in one view and read off the position in mm. Note that the position in mm is a configurable option - the default position is the top-left corner of the view, but it may have been moved or removed altogether by a user. Click here for help on restoring it if necessary.
Once you have noted down the X,Y and Z components of the position, they can be entered into the top line of the calculator.
For a hip replacement, the process is the same, however a metal-on-metal or ceramic-on-ceramic head may present problems locating the outline of the head itself. To aid in this, if the head size is know, a circle of that size can be drawn and placed over the head in each view, aligning it with visible parts of the outline external to the cup to estimate the centre.
The proximal diaphyseal centre is positioned relative to the lesser trochanter. We have taken a point 20mm below the centre of the lesser trochanter as our starting position to set the femoral frame of reference.
First align the longitudinal axes with the axis of the femoral shaft.
In this example, it is assumed that OsiriX is being used to make the measurements. Other CT measurement software is available, but the screen layout and features available may be different.
Locate the centre of the lesser trochanter by moving the axial slice position up or down until it is clearly in view.
Once the lesser trochanter is located, Place the mouse pointer over the centre of the shaft at the height of the lesser trochanter and make a note of the Z position (height) of the lesser trochanter.
Now move the axial slice position distally by 20mm:
On the slice 20mm distal from the lesser trochanter, locate the centre of the shaft with the cross-hairs. Drawing a circle around the outline of the shaft may make finding the centre easier, and zooming in will make mouse positioning more accurate.
Now place the mouse pointer over the cross-hair intersection on the axial view and note down the position in mm.
Once you have noted down the X,Y and Z components of the position, they can be entered into the second line of the calculator.
The distal diaphyseal centre is positioned relative to the proximal diaphyseal centre. For our protocol in this example, we have taken a distance of 100mm vertically between the diaphyseal centres.
In this example, it is assumed that OsiriX is being used to make the measurements. Other CT measurement software is available, but the screen layout and features available may be different.
Drop the axial slice position 100mm from that recorded for the proximal diaphyseal centre
On the axial slice locate the centre of the shaft with the cross-hairs. Drawing a circle around the outline of the shaft may make finding the centre easier, and zooming in will make mouse positioning more accurate.
Now place the mouse pointer over the cross-hair intersection on the axial view and note down the position in mm.
Once you have noted down the X,Y and Z components of the position, they can be entered into the third line of the calculator.
Locate the knee slices - depending on the scanning protocol used and the software used to display the data, the knees may be in a different series and may be displayed separately from the hips. In this example, the knees have been recorded in the same series as the hips.
In this example, it is assumed that OsiriX is being used to make the measurements. Other CT measurement software is available, but the screen layout and features available may be different.
Locate the medial epicondyle, and place the cross-hairs onto it - here we have used the anatomical medial epicondyle by finding the apex.
Now place the mouse pointer over the cross-hair intersection and note down the position in mm.
Once you have noted down the X,Y and Z components of the position, they can be entered into the fourth line of the calculator.
Locate the knee slices - depending on the scanning protocol used and the software used to display the data, the knees may be in a different series and may be displayed separately from the hips. In this example, the knees have been recorded in the same series as the hips.
In this example, it is assumed that OsiriX is being used to make the measurements. Other CT measurement software is available, but the screen layout and features available may be different.
Locate the lateral epicondyle by finding the apex on the lateral side, and place the cross-hairs onto it
Now place the mouse pointer over the cross-hair intersection and note down the position in mm.
Once you have noted down the X,Y and Z components of the position, they can be entered into the fifth line of the calculator.
OsiriX is user configurable in how it displays annotation overlays. For the measurements used here, it is simplest to obtain them using the mouse position in mm, which is normally displayed in the top-left corner of the active view. However, it is quite possible that another user may have switched off annotations - to get them back, click the OsiriX option in the top-line menu and select Preferences.
In the preferences menu select Annotations.
The easiest way to get the mouse mm measure back is to press reset at the top of the annotations screen. After updating the annotations, you may have to close the current views and re-open them to show the updated settings.
If the annotations are still not shown after resetting them as above, in the 2D Viewer menu, select the Annotations option and ensure that either 'Basic' or 'Full' are ticked.