Introduction
There are a number of relevant investigations for hip and knee pain including: X-ray, MRI, bone scan, CT scan, ultrasound and other special scans. Each of these have their own indication and are ordered for different reasons. Not all patients will have the same investigations.
X-Ray
An X-Ray is the most important investigation and almost provides a large amount of useful information to the doctor. Almost every patient who is referred to an Orthopaedic surgeon for a hip or knee problem should have an X-Ray. A number of patients bring an X-Ray report with them. This is NOT as helpful as seeing the actual pictures themselves because Orthopaedic surgeons are looking for particular things that radiologists may not be aware of.
For a knee the standard set of X-Rays are a weight bearing AP (This should be an 18 inch film) a lateral, a notch view and a skyline patellar view. These will need to be asked for specifically on the x-ray request form. There are other special X-Ray views which will be done if it is noted noting on the request form that you are looking for a particular diagnosis. The reason for X-Raying most patients is that occasionally there are surprising findings such as a tumour, loose body or foreign body which patients are not aware of (also stress fractures and developmental disorders).
In general, X-Rays look at the bony architecture, the mechanical alignment, any soft tissue swelling, effusions and fractures as well as other more rare disorders.
X-rays in Total Knee Replacement
Regular follow up X-Rays are necessary once a patient has had a joint replacement. The surgeon looks at alignment, the type of joint, wear, the patella (including the position and any loosening), loosening of the prosthesis or any loose bodies. In general, patients should have check X-Rays every one or two years depending on the type of joint and the age of the patient.
Tendon and Ligament Calcification
X-Rays are also useful for looking at calcification in the soft tissues including diagnoses such as Jumper’s knee, Osgood Schlatter’s disease, heterotopic ossification, myositis ossificans and chondrocalcinosis.
CT Scan
A CT scan is occasionally ordered to look for bony abnormalities such as a fracture or a tumour. It is not needed for arthritis or any soft tissue disorders such as meniscal tears or ligament ruptures. A CT Scanogram (to measure leg lengths) is of some limited benefit but a clinical examination is often more useful. CT scans are also useful looking at patellar tracking, in which case a special request needs to be ordered looking at different degrees of flexion, usually from 0-45 degrees.
Ultra Sound
Ultrasounds are of very little benefit in the knee or the hip joint. They are being ordered quite regularly and are generally of very little benefit. The only time they are of use is when looking for a rupture of the extensor mechanisms such as in patient who can’t straight leg raise and you suspect may have a rupture of the patellar tendon or the quadriceps tendon. The other indication is looking for a foreign body. They are not useful when looking at the cruciates, the collaterals, or menisci.
Bone Scan
A bone scan is useful if hips and knees in looking for unexplained pain, occult fractures, osteonecrosis, occasionally osteoarthritis, tumours, and infection.
MRI Scan
MRI’s are being increasingly ordered and patients are often asking for these. They are very useful in the patient you suspect has a soft tissue injury such as a meniscal tear, cruciate ligament injury or collateral ligament injury. It is also of great benefit in osteonecrosis in looking at the size of the lesion and any articular surface collapse. It is also useful for tumours, stress fractures and occasionally looking for chondral damage although it is not as useful as an arthroscopy in this diagnosis. It is also helpful, especially in workers compensation cases to document the injury as well as to speed approval of the operation. The problem with MRI scans is that they sometimes can find a false positive which can make the patient concerned unnecessarily.
In particular, for cruciate ligament ruptures, they are useful in a patient who cannot relax or a patient who has a leg which is too large to examine. It is also useful in planning surgery, especially looking for meniscal tears which may need to be repaired.
Osteonecrosis
It is useful in confirming the diagnosis, looking at the size and the position of the lesion as well as looking for articular surface collapse.
MRI’s are also useful in looking at the extensor mechanism, especially if the ultrasound is equivocal. The classic patient is a patient who has a tender area over the extensor mechanism with a palpable defect and is unable to straight leg raise. MRI’s are not particularly useful in arthritis but can look at the meniscus.
Other Scans
Colloid and White Cell Scans are useful in looking for infection, particularly around prostheses.
Hip Investigations
X-Rays of the hip should again be ordered in any patient who has any hip pain referred to an Orthopaedic surgeon, particularly those with arthritis. The X-Rays should be AP of the pelvis centered over the pubis and AP and lateral of the hip. Occasionally there are special views but these only need to be ordered by the Orthopaedic surgeon. X-Rays look for arthritis, soft tissue calcification fractures, infection, avascular necrosis and joint replacements.
CT scans of the hip are useful looking for stress fractures or occult fractures following trauma. They are also useful in looking for loose bodies as well as tumours and defining the anatomy, particularly of the acetabulum when planning any surgery.
MRI’s are useful in hip pain, particularly looking for labral tears. They are also useful in diagnosing osteonecrosis, particularly the size of the lesion and articular surface collapse. They are also helpful for infection, tumours and fractures.
Hip Injections – Injections into the hip joint can assist with a diagnosis of arthritis because, if the pain is alleviated by steroid injection and local anaesthetic, this means that the pathology is coming from the hip. This is particularly useful when trying to differentiate between pain in the back and the hip.
Hip Bone Scans are useful looking from fractures, arthritis, tumours and metabolic bone disease.
White Cell Scans can be useful in looking for septic arthritis, osteomyelitis and infected joints.
