Screw Home Mechanism of The Knee Joint

Original Editor - Rafet Irmak Top Contributors - Rafet Irmak and Kim Jackson


Screw home mechanism (SHM) of knee joint is a critical mechanism that play an important role in terminal extension (last 15 degreas) of the knee [1][2][3][4][5][6]. Knee joint is a hinge type, uniaxial joint that allows flexion and extension movements. However there is a slight rotation in last 15 degrees of knee extension due to inequality of articular surface of femur condyles.

SHM is a result of Vastus Medialis Obliquus and Vastus Lateralis Obliquus muscle function. The SHM improves the mechanical advantage of knee joint to perform full terminal extension then locks the knee joint[7]. Popliteus muscle unlocks the knee.

SHM is a normal characteristics of knee[8][9][10]. In most musculoskeletal problems of knee, theinsufficiency of terminal extension and SHM is a common problem[11][12][6].

SHM Related Anatomy of Knee Joint

Lane Medical Center, Basset Image Collection is a unique non commercial online anatomy image database. The selected dissection photo list is below. In these dissection photos please identfy the sutructures below for better understanding of the SHM.

Condyles of Femur. Interior of right knee joint, anterior view. Image #190-4

Interior of right knee joint, anterior view

2 . Lateral condyle of femur

3 . Lateral meniscus

5 . Patellar articular surface

6 . Quadriceps femoris muscle (reflected)

9 . Intercondylar fossa

10 . Medial condyle of femur

12 . Medial meniscus

Interior of right knee joint, anterior cruciate ligament

Interior of right knee joint, anterior cruciate ligament. Image #190-5

1 . Patellar femoral surface

2 . Lateral epicondyle

3 . Lateral condyle of femur

4 . Superior articular surface of tibia

5 . Lateral meniscus

7 . Patellar articular surface

9 . Anterior cruciate ligament

10 . Medial condyle of femur

11 . Medial meniscus

Dissection of anterior and medial aspects of thigh

Quadriceps femoris muscle. Image #188-7

4 . Vastus lateralis muscle (retracted laterally)

5 . Rectus lemons muscle (tendon of insertion)

6 . Patella

7 . Patellar ligament

8 . Rectus femoris muscle (tendon of origin)

12 . Vastus intermedius muscle

13 . Vastus medialis muscle

Also you can check anatomy of knee section in Physiopedia.

Physiological/Mechanical Movement Patern

Mechanical Paradox: Articular surface of medial condyle of femur is greater than the articular surface of later condyle.

Articular surface paradox of femur

Physiological Movement: Both medial and lateral condyle moves on articular surface of tibia (medial/lateral meniscus)

Kinetic Chain: Open kinetic chain (foot and calf freely moves)

Movement: Terminal extension of the knee

Screw Home Movement: During extension when all articular surface of lateral condyle is used by roll movement there are still unused articular surface on medial condyle. Femur glides posteriorly on tibia to use full articular surface of medial condyle. Then knee is locked by vastus medialis obliquus.

Clinical Manifestations of Screw Home Mechanism

Patellofemoral Pain Sydrome

Osteoartritis of the Knee

Terminal Knee Extention Exercise

Terminal knee extention is a specific exercise for vastus medialis obliquus muscle. In earlier descriptions of the exercise it is sugested to improve muscle force of vastus medialis obliquus. Further descriptions also focused on motor leraning and vastus medialis obliquus time delay during terminal extention.

VMO terminal extension exercise. FromRehab My Patient

Clinical Relevelance

In most of the knee problems generally there is an insufficiency on terminal extension and vastus medialis obliquus function. Restoration of terminal extension is an important goal of rehabilitation program. Terminal extension exercises like interventions have important contribution on restoration of screw home movement /terminal extension.


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