Knee joint

Knee joint

Two projections are taken routinely:

• anterio-posterior (AP)

lateral.

Knee joint

The AP projection is frequently obtained in the weight-bearing/standing position as it provides more meaningful information on the condition of the joint when compared to the traditional (supine) non-weight-bearing technique

Antero-posterior – weight-bearing

Position of patient and image receptor

• The patient stands with their back against the vertical Bucky or DDR receptor (grid removed) using it for support if necessary.
• The patient’s weight is distributed equally.
• The knee is rotated so that the patella lies equally between the femoral condyles.
• The centre of the image receptor is level with the palpable upper borders of the tibial condyles.
• This projection is useful to demonstrate alignment of the femur and tibia in the investigation of valgus (bow-leg) or varus (knock-knee) deformity. Any such deformity will be accentuated when weight-bearing, which more closely resembles the real-life situation. It is commonly requested to assess alignment prior to joint replacement, as narrowing of one side to the joint space more than the other will produce varus or valgus tilt.

Direction and location of the X-ray beam

• The collimated horizontal beam is centred 1 cm below the apex of the patella through the joint space, with the central ray at 90° to the long axis of the tibia (midway between the palpable upper borders of the tibial condyles).

Essential image characteristics (Figs above)

• The patella must be centralised over the femur.
• The image should include the proximal 1/3 of the tibia and fibula and distal 1/3 of the femur.

Antero-posterior – supine

Position of patient and image receptor

• The patient is either supine or seated on the X-ray table or trolley, with both legs extended.
• The affected limb is rotated to centralise the patella between the femoral condyles, and sandbags are placed against the ankle to help maintain this position.
• The image receptor, i.e. 18 × 24 CR cassette should be in close contact with the posterior aspect of the knee joint, with its centre level with the upper borders of the tibial condyles.

Direction and location of the X-ray beam

• The vertical collimated central beam is centred 1 cm below the apex of the patella through the joint space, with the central ray at 90° to the long axis of the tibia (midway between the palpable upper borders of the tibial condyles).

Lateral (Basic)

• Position of patient and image receptor
• The patient lies on the side to be examined, with the knee flexed at 45° or 90°.
• The other limb is brought forward in front of the one being examined and supported on a sandbag.
• A pad is placed under the ankle of the affected side to bring the long axis of the tibia parallel to the image receptor. Dorsiflexion of the foot helps maintain this position.
• The position of the limb is now adjusted to ensure that the femoral condyles are superimposed vertically.
• The medial tibial condyle is placed level with the centre of the receptor.
• An alternative method is to keep the unaffected limb behind the knee being examined with the ankle flexed and the heel resting on the lower shaft of the unaffected leg.

Direction and location of the X-ray beam

• The collimated vertical beam is centred to the middle of the superior border of the medial tibial condyle, with the central ray at 90° to the long axis of the tibia.

Essential image characteristics (Fig. above)

• The patella should be projected clear of the femur.
• The femoral condyles should be superimposed.
• The proximal tibio-fibular joint is not clearly visible.(Approximately 1/3 of the fibula head should be superimposed behind the tibia.)

Femur – shaft

Two projections are taken routinely, preferably with both the knee and hip joints included on the image.

• Antero-posterior

Lateral

Antero-posterior (Basic)

Position of patient and image receptor

• The patient lies supine on the X-ray table, with both legs extended and the affected limb positioned to the centre line of the table.
• The affected limb is rotated to centralise the patella over the femur.
• Sandbags are placed below the knee to help maintain the position.
• The image receptor/Bucky mechanism is located directly under the posterior aspect of the thigh to include both the hip and the knee joints.
• Alternatively, a CR cassette or mobile DDR detector is positioned directly under the limb, against the posterior aspect of the thigh to include the knee and hip joints.

Direction and location of the X-ray beam

• The collimated vertical beam is centred to the mid-shaft of the femur, with the central ray at 90° to an imaginary line joining both femoral condyles.

Essential image characteristics

• Ideally, the length of the femur should be visualised, including the hip and knee joints. This may be difficult to obtain and an additional projection of the knee or hip joint may be required if coverage is not initially achieved; however, this will depend on the clinical information required.
• The patella should be centralised to indicate rotation has been minimised.

Direction and location of the X-ray beam

• The collimated vertical beam is centred to the middle of the femoral shaft, with the central ray parallel to the imaginary line joining the femoral condyles.

Lateral (Basic)

Position of patient and image receptor

• From the antero-posterior position, the patient rotates on to the affected side with the knee is slightly flexed and the patient adjusted so that the thigh is positioned to the centre line of the table.
• The pelvis is rotated backwards to separate the thighs.
• The position of the limb is then adjusted to superimpose the femoral condyles vertically.
• Pads are used to support the opposite limb behind the one being examined.
• The image receptor/Bucky mechanism is located directly under the lateral aspect of the thigh to include the knee joint and as much of the femur as possible.
• Alternatively, a CR cassette or mobile DDR detector is positioned directly under the thigh to include the knee and hip joints

Essential image characteristics

• The length of the femur should be visualised, including the hip and knee joints.
• Often, an additional projection of the hip joint using a grid,may be required if coverage is not initially achieved or the image quality is affected by scatter and/or noise in the proximal femur; however, this will depend on the clinical information required and the patient size.

Radiation protection/dose

• Careful technique and close collimation will assist in reducing the patient dose.