Differentiating Patellofemoral and Tibiofemoral Pain
- 1 Introduction
- 2 Brief Anatomy of the Knee
- 3 Common Causes of Knee Pain
- 4 Interview/ Subjective Examination
- 5 Objective Examination
- 6 Conclusion
- 7 References
Pain experienced in and around the knee is a widespread occurrence. The knee is made up of the tibiofemoral and patellofemoral joint. Pain experienced in the knee can occur from several different structures as well as causes. Performing a thorough, detailed interview and physical examination can assist you in clinically reasoning the various differential diagnoses and isolating the exact cause of symptoms.
Brief Anatomy of the Knee
The tibiofemoral joint occurs where the femur meets the tibia. It includes intra-articular structures such as the menisci and cruciate ligaments (ACL and PCL) and extracapsular structure such as the collateral ligaments (MCL and LCL).
The patellofemoral joint is where the patella, a triangular sesamoid bone, articulates with the femur. The patella acts as a pulley to increase the lever arm of the quadricep muscles. The infrapatellar fat pad lies just inferior and under the patella.
Common Causes of Knee Pain
- Patellofemoral Pain Syndrome
- Patellar Tendinopathy
- Patellar Dislocation
- Infrapatellar Fat pad impingement
- Patellofemoral Osteoarthritis
- Ligamentous sprains (ACL, PCL, MCL, LCL)
- Medial meniscus tear
- Articular Cartilage Injury
- Tibiofemoral Osteoarthritis (also commonly referred to as Knee Osteoarthritis)
Interview/ Subjective Examination
The older a person is, the more likely it is that there may be arthritic changes within the knee that are contributing to the problem.
Area of Pain
The area of pain is significant in the knee. Pain under or around the patella, that is not so easy to pinpoint, is often more indicative of patellofemoral pain. In contrast, pain localised to the inferior pole of the patella can be more indicative of infrapatellar fat pad pathology or patellar tendinopathy. After an acute traumatic injury, the area of pain can help to determine which structures are damaged. For example, pain on the medial tibiofemoral joint line associated with a history of a valgus stress to the knee would implicate the MCL
Mechanism of injury
Knee pain that has an insidious onset is more commonly found in patellar tendinopathy, patellofemoral pain syndrome (especially if the pain is predominantly anterior) and osteoarthritis. Careful questioning around the changes in activity can assist with clinical reasoning around the structures involved. Someone who has recently increased their load with repetitive loading activities such as jumping, for example, a volleyball player would be at risk of patellar tendinopathy. Whereas repetitive movements into end range extension without excessive load such as kicking in swimming would potentially aggravate the fat pad..
A traumatic injury is more likely to result in injury to the ligamentous, meniscal or osteochondral injury to the knee. Careful questioning around the exact direction of the force can help determine which structures were potentially compromised. The cruciate ligaments tend to be vulnerable with rotation, and end range forced extension (ACL) or end range forced flexion (PCL). Twisting injuries, especially with a fixed foot, often result in meniscal tears with or without associated ACL injuries. 
If an audible pop or snap is heard at the time of injury, the ACL is very likely to have been injured. A dislocated patella is another common finding when an audible pop was heard during injury.
Speed of Swelling
How fast and the extent to which the knee swells following a traumatic incident can be a good indicator of the structures that are involved in the injury. Immediate, significant swelling that occurs within an hour to 2 hours is a sign of intra-articular swelling or a haemarthrosis. Immediate swelling is often an indication of significant injury to the intra-capsular structures such as the ACL or a fracture. An effusion that develops 6-24 hours later is more often a sign of meniscal and chondral injuries with collateral ligament injuries often having minimal swelling if any.
Ability to Continue with Activity
If the injured person can play on after sustaining an injury to the knee, this is often a sign of a less significant injury compared to if they are unable to weight-bear and have to be carried off the field.
Anterior knee pain that worsens during activity is more often a sign of patellofemoral pain, whereas patellar tendinopathy usually warms up during activity and may flare up afterwards. An increase of pain at the patellar tendon with dose-dependent loading is a classic sign of patellar tendinopathy.
Giving way is a commonly complained about symptom in the knee. True-giving-way, where the knee actually collapses, is usually a sign of ligamentous laxity and normally a compromised ACL. Pseudo-giving-way, which is more of a feeling that the knee is going to give way rather than an actual collapse, is a sign of poor dynamic control of the quads either from weakness or pain inhibition 
Clicking and Locking
True-locking, where the knee cannot move past a certain point, is often a sign of an intra-articular loose body or a bucket handle tear of the meniscus. By moving the knee in various directions, they may be able to “unlock” the knee indicating something has moved within the joint. True locking requires immediate referral for arthroscopic surgery. Pseudo-locking is when the knee is unable to extend or flex from stiffness or pain inhibition. It can occur in tibiofemoral injuries but may also occur in the knee where the patella does not engage properly with the trochlear
Once the interview is complete, you should have enough information to clinically reason
Beginning the physical examination with a general and local observation is the best way to get a general picture of the knee. Observation should be done in standing as well as supine. It is helpful to look for any swelling, bruising or obvious physical deformities
In standing the general alignment of the lower limb can be assessed. A genu varum can occur if there is a decrease in the joint space of the medial tibiofemoral joint or varus can indicate a narrowing of the lateral tibiofemoral joint space. The position of the patella can be observed in standing and supine.
If there is any swelling around the knee it may be helpful to identify if it is the whole joint or just a specific part such as around the infra-patellar fat pad. Muscle mass can be compared with the other leg in standing and supine to determine if there is any atrophy or hypertrophy of specific muscles.
Range of Movement
Assessing the active and passive physiological movement of the knee joint is essential to a thorough examination of the knee. The range of motion, the quality of movement, the end feel, is the patient scared or anxious through movement or is there any guarding are all crucial factors that need to be assessed and will help formulate a comprehensive clinical picture.
Palpation of knee structures can be useful to help differentiate the source of the symptoms. Palpation should be used with caution; however, just because a structure is painful with palpation, does not mean that that structure is the cause of the pain. Joint line tenderness has been used to aid in the diagnosis of meniscal and articular cartilage lesions ; however, it has also been shown that people with PFPS have joint line tenderness on palpation. Interpretation of painful structures on palpation should be used with caution. A good question to ask when a structure is painful on palpation would be “is that YOUR pain”
Palpation can be useful in the identification of inflammatory reactions within the knee. A knee that is warm to touch may indicate an active inflammatory process from a new injury, active osteoarthritis or systemic inflammatory disorder. Using information gathered in the interview will help determine which category they fall into
Special tests of the Knee
There is an extensive number of knee special tests. After a thorough subjective examination, the physical examination and careful selection of appropriate tests should be used to confirm the hypothesis.
A single test alone cannot diagnose conditions in the knee, but rather clusters of clinical findings should be used to form a complete clinical picture
Special tests of the knee are influenced by many factors that affect the clinical and diagnostic accuracy of the tests – the experience of the examiner, muscle spasm or guarding, as well as swelling and the extent of the injury, all play a role.
A systematic review conducted in 2015 found that the sensitivity and specificity of the Lachman and Pivot shift test to diagnose ACL insufficiency were dependent on if the patients were awake or anaesthetised as well as whether it was a partial or complete tear..
Smith et al (2015) conducted a systematic review on using the diagnostic accuracy for meniscal injuries using the McMurray's, Apley's, Joint Line Tenderness and Thessaly. They concluded poor diagnostic accuracy however they do mention that the quality of studies involved was generally poor and these results should be interpreted with caution
In the research, special tests have a wide variance in specificity and sensitivity. The importance of a knee examination is to use a collection of information rather than an interpretation of a single test in isolation.
When performing a differential diagnosis of the knee, it is important to look at the whole clinical picture, using information from the interview as well as the physical examination. It is important to differentiate where the symptoms are arising from as well as any other associated factors that may be contributing to the problem.
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