Surgical Intervention of the Hip and Knee 

Topic three has two broad themes; why do we need to surgically replace joints and how do we plan and achieve these interventions? It will cover the clinically relevant pathology that leads to patients require joint replacement surgery, mainly osteoarthritis of both the hip and the knee. Additionally, this topic will cover how we plan joint replacement surgery in both the primary and the revision setting to achieve optimal positioning.


Osteoarthritis

Osteoarthritis is a disease of hyaline cartilage, the 3mm lining of soft connective tissue which covers the epiphysis of the bone at synovial joints. This is a very common disease, with the majority of the population having radiological signs by the age of 60, with around 25% being symptomatic. The main symptoms is pain in the affected joint.

Symptoms in osteoarthritis are caused when the cartilage of a joint degenerates, and the subchondral bone (beneath the cartilage) begins to articulate. The cartilage is aneural (lacking sensory nerve endings) whereas the subchondral bone has a lot of nerve endings, causing the pain felt in the disease. 

Osteoarthritis can be primary, with an unknown cause, thought to be related to a number of factors (genetic, metabolic, biochemical and biomechanical). This is the most common type. 

Secondary osteoarthritis is when something leads to the osteoarthritis forming. For example, trauma to the knee joint such as a torn meniscus can damage the underlying hyaline cartilage and increase the risk of developing osteoarthritis later on in life. 

Another example of secondary osteoarthritis, outlined by the case below, is developmental dysplasia of the hip. This is where the hip joint does not develop correctly during foetal development. The change in the structure of the joint increases the risk of developing secondary OA.

Note that osteoarthritis isn't the only arthritis that can affect the joints. Inflammatory arthritis can affect the synovium of the joint, increasing inflammatory mediators within the the joint, speeding up the process of cartilage degeneration, leading to a similar final pathology.

The below image shows the radiographic changes associated with osteoarthritis. LOSS is a good acronym - Loss of joint space (causing radiographic narrowing as cartilage is radiolucent), osteophytes (bony outgrowths), sclerosis of the subchondral bone (thickening of the bone at the end of the joints making it more dense and therefore whiter on radiographs) and subchondral cysts (fluid filled cysts just below the subchondral bone).

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EXAMPLE CASES - Click on the image to load the case

The left hip demonstrates all four of the LOSS signs of osteoarthritis due to developmental dysplasia of the hip.

This is an interesting case, there are clear radiological signs of osteoarthritis, but do they need a hip replacement? The answer lies in the patients functionality and their symptoms.

Does this patient need a hip replacement? Once again, clear arthritis of the joint, but how are they symptomatically? Take a look at the case.

Which hip shows signs of OA? This is more subtle than the other three cases. This case also overlaps with the second aspect of topic three, surgical planning.


Surgical planning

Hip and knee replacement surgery often uses components which are taken from the shelf i.e. standard components. Even with these components, the anatomy of the joint differs between patients, so their operation needs to be planned. 

Replacing a joint isn't as simple as taking one out and inserting a new one. The reconstruction needs to ensure the length of the patients leg remains the same and the mechanics of the joint are not adjusted. Malpositioned replacement joints can lead to a range of complications, as we will explore in topic four.

Surgical planning can involve planning for the use of standard components, planning to produce instrumentation to help position components and planning to make totally custom implants. All surgical planning relies on pre-operative imaging, as covered in topic two. The cases below highlight some of these planning methods.

EXAMPLE CASES - Click on the image to load the case:

This case looks at patient specific instrumentation in revision surgery. Pre-operative CT scans are used to design instruments which can be used intra-operatively to guide the surgeon in achieving the optimal positioning of the implant.

This is one slice from a CT scan which was used to model the patients pelvis, design a custom implant and design custom instrumentation to fit the implant.

This 3D computer model has been recreated from a pre-operative CT scan using specialist computer software. These reconstructions are used by surgeons and biomedical engineers to design implants.

Another very complex CT scan used to design a custom implant. This was a highly complex case as a proximal femoral replacement implant was also required.