Anatomy
Lower Limb
The 'screw home' mechanism involved in locking of the knee joint refers to which of the following actions:
Answer:
When standing, the knee joint is 'locked' in position to reduce the amount of muscle work needed to maintain the standing weight bearing position. This locking mechanism occurs partly due to the change in the shape/size of the articulating femoral surfaces (in the flexed position, the surfaces of the femoral condyles that articulate with the tibia are curved/round, but in extension, the surfaces are flat, and consequently the joint surfaces become larger and more stable in extension) and partly due to medial rotation of the femur on the tibia in full extension; medial rotation and full extension tightens all the associated ligaments (the screw home mechanism).Knee Joint
Anatomy / Lower Limb / Popliteal Fossa and Knee
Last Updated: 10th April 2019
Table: Anatomical Overview of the Knee Joint
| Joint | Knee |
|---|---|
| Type | Modified hinge synovial joint |
| Articulations | Femoral condyles with tibial condyles (tibiofemoral articulation) and patella with anterior femur (patellofemoral articulation) |
| Stabilising factors | Fibrous capsule, tibial spines, menisci, tibial/fibular collateral ligament, anterior/posterior cruciate ligament, vastus medialis and lateralis muscles, oblique popliteal ligament, iliotibial tract, muscle tendons (hamstrings, gastrocnemius, sartorius, gracilis) |
| Movements | Flexion/Extension, Medial/Lateral rotation in flexed position |
Joint Articulations
The knee joint is formed from two articulations:
- the main weight bearing tibiofemoral articulation between the two femoral condyles and the adjacent surfaces of the superior aspect of the tibial condyles
- the patellofemoral articulation between the anterior femur and the patella which allows the pull of the quadriceps femoris muscle to be directed anteriorly over the knee to the tibia without tendon wear
Knee Joint. (Image by OpenStax College [CC BY 3.0 , via Wikimedia Commons)
Joint Movements
The knee joint is a modified hinge synovial joint, allowing mainly flexion and extension, but also a small degree of medial and lateral rotation.
Table: Movements of the Knee Joint
| Movement | Main Muscles Involved | Main Nerves Involved |
|---|---|---|
| Flexion | Hamstrings, Gracilis, Sartorius, Gastrocnemius, Plantaris | Sciatic nerve, Femoral nerve, Obturator nerve |
| Extension | Quadriceps femoris | Femoral nerve |
‘Screw-home’ Mechanism and ‘Locking’
When standing, the knee joint is 'locked' in position to reduce the amount of muscle work needed to maintain the standing weight bearing position. This locking mechanism occurs partly due to the change in the shape/size of the articulating femoral surfaces (in the flexed position, the surfaces of the femoral condyles that articulate with the tibia are curved/round, but in extension, the surfaces are flat, and consequently the joint surfaces become larger and more stable in extension) and partly due to medial rotation of the femur on the tibia in full extension; medial rotation and full extension tightens all the associated ligaments (the screw home mechanism). Contraction of the popliteus muscle 'unlocks' the knee by initiating lateral rotation of the femur on the tibia, and allowing flexion.
Joint Capsule
The fibrous membrane of the knee joint is reinforced anteriorly by the tendinous expansions of the vastus lateralis and vastus medialis muscles, anterolaterally by a fibrous extension from the iliotibial tract and posteromedially by the oblique popliteal ligament, an extension from the tendon of the semimembranosus muscle (the oblique popliteal ligament resists hyperextension and lateral rotation of the leg). The upper end of the popliteus muscle passes through an opening in the posterolateral aspect of the fibrous membrane of the knee.
Menisci
The two menisci are C-shaped fibrocartilaginous structures that lie between the femoral condyles and the tibia, attaching at each end to facets in the intercondylar region of the tibial plateau. In addition, the medial meniscus is also attached around its margin to the joint capsule and to the tibial collateral ligament, unlike the smaller, more mobile lateral meniscus. This means any damage to the tibial collateral ligament results in tearing of the medial meniscus. The menisci deepen the articular surface of the tibia increasing stability of the joint, improve congruence between the femoral and tibial condyles during joint movements and play an important role in shock absorption.
Menisci. (Image by OpenStax College [CC BY 3.0 , via Wikimedia Commons)
Ligaments
COLLATERAL LIGAMENTS:
The tibial collateral ligament is attached proximally to the medial epicondyle of the femur and distally to the medial tibia. The fibular collateral ligament is attached proximally to the lateral condyle of the femur and distally to the lateral fibula. The tibial and fibular collateral ligaments act to stabilise the knee joint medially and laterally respectively, limiting extension and preventing adduction and abduction movements. The tibial collateral ligament is also attached to the medial meniscus; this means any damage to the tibial collateral ligament usually results in tearing of the medial meniscus.
CRUCIATE LIGAMENTS:
The cruciate ligaments interconnect the adjacent ends of the femur and tibia and maintain their opposed positions during movement.
- The anterior cruciate ligament (the weaker of the two) attaches to the anterior part of the intercondylar area of the tibia and ascends posteriorly to attach to the lateral wall of the intercondylar fossa of the femur and acts to prevent anterior displacement of the tibia relative to the femur. The ligament is lax during flexion and taut during extension thus it may be torn when the knee is hyperextended (or by the application of a large force to the back of the knee with the joint partly flexed). The anterior drawer sign may be seen where there is forward sliding of the tibia on the femur.
- The posterior cruciate ligament (the stronger of the two) attaches to the posterior part of the intercondylar area of the tibia and ascends anteriorly to attach to the medial wall of the intercondylar fossa of the femur and acts to prevent posterior dislocation of the tibia relative to the femur. The ligament is lax during extension and taut during flexion and thus it may be torn in a hyperflexion injury, where a large force is applied to the tibia when the knee is flexed. The posterior drawer sign may be seen where there is backward sliding of the tibia on the femur.
KNEE LIGAMENT INJURY:
The 'unhappy triad' typically occurs due to a lateral force to an extended knee, e.g. in a football tackle. It refers to injury of the anterior cruciate ligament (due to forward displacement of the tibia), the tibial collateral ligament (due to excessive abduction) and the medial meniscus (due to its attachment on the tibial collateral ligament).
Bursae
The synovial membrane of the knee joint forms pouches in two locations to provide low-friction surfaces for the movement of tendons associated with the joint:
- The subpopliteal recess - extends posterolaterally and lies between the lateral meniscus and the tendon of the popliteus muscle
- The suprapatellar bursa extends superiorly between the distal end of the shaft of the femur and the quadriceps femoris muscle and tendon
Other bursae associated with the knee, but not normally communicating with the synovial joint, include the subcutaneous prepatellar bursa, the deep and subcutaneous infrapatellar bursae separated by the patella ligament, and numerous other bursae associated with tendons and ligaments around the knee joint. Housemaid's knee is inflammation of the prepatellar bursa, and Clergyman's knee is inflammation of the subcutaneous infrapatellar bursa.
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
