Roentgen Stereophotogrammetric Analysis (RSA) is a radiographic technique (see the document RSA technique) with high accuracy used in orthopedic field for measuring micromotion at bone/prosthesis interface or for kinematical studies in general. In our lab RSA was used and it is still used both in traditional set-ups, to assess stability of new prostheses or to compare stability properties between different design prostheses, and in non standard conditions for non conventional studies. We have a database comprehending more than 150 patients, implanted with different kinds of prostheses and with a range of exams from post-operative tests  till 10 year follow-ups.
A few examples of ongoing studies are following.

Study on the stability of mobile meniscal Stryker Howmedica© prostheses

A group of 25 patients treated with mobile bearing total knee PCR prosthesis underwent a 3 year follow-up series, using a traditional RSA protocol. It means that patients underwent radiographic tests  immediately after the operation (reference exam) and periodically (3-6-12 months and then yearly), putting  the treated knees into a calibration cage (see the image aside).

For this study tibial components of the prostheses were marked on purpose by the manufacturer, attaching 3 tantalum markers, one at the bottom of stem and two under the tibial baseplate, close to the stem (see the image below).

It is possible to measure the migration, i.e. the irreversible motion of the prosthesis relative to the bone, through comparison between the reference examination and the following exams.  The migration is quantified by the maximum total point motion (MTPM) parameter, i.e. the displacement between consecutive exams of the prosthesis marker which moved most. The first two year follow-up data give already an indication about migration pattern: MTPM values higher than 0.2mm can be considered consistent with an early prosthesis mobilization.

Aanlysis of the mobility of meniscus in mobile meniscal Stryker Howmedica© prostheses through weight-bearing examinations

The goal is studying by RSA technique the effective movement of the polyethylene meniscus in a mobile-bearing prosthesis trough weight bearing tests. 
The manufacturer provided the PE inlay component with 5 tantalum micro spheres embedded in it.
At yearly repeated follow-ups the used protocol provides another series of exams besides the supine ones: they are executed standing and weight-bearing and they simulate mechanical stress occurring during daily life:

PWB (Plain Weight-Bearing): the patient is standing and overloads the limb under examination (see the picture aside).

MIR (Maximal Internal Rotation): same conditions as PWB, but with the foot of the loaded limb in full internal rotation 
MER (Maximal External Rotation): it differs from MIR only because the patient's foot is kept in full external rotation.
SQT (Squatting): the patient squats on the loaded leg until the knee reaches 60-70° of flexion.
IRT (Internally Rotating Torque): the patient must oppose the internally rotating torque of a rotating plate on which a 3 Kg  weight  is attached.
ERT (Externally Rotating Torque): equivalent to IRT, but with the rotating plate loaded to induce an externally rotating torque.

The comparison between the exam in the supine position and PWB and between PWB and the weight bearing tests belonging to the same control allow the stress inducible displacement to be assessed, that is the reversible motion of the prosthesis relative to the bone induced by an external force.

Concluded studies

• 2005 - Analysis about the stress inducible displacement in unicompartmental DURACON UNI prostheses (16 patients) (see the image aside).
• 2002 - Study on the stability of unicompartmental DURACON UNI prostheses (20 patients).
• 2001 - Study on the stability of half bearings Interax prostheses (18 patients).
• 2000 - Kinematics studies of proximal tibio-fibular joint (cadaveric specimen).