Anatomy Series: The Knee

by Brie Eteson, Medical Communications, Digital Surgery

In our anatomy series, Touch Surgery™ brings you the need-to-know anatomical knowledge for medical practitioners. A good foundation in anatomy is essential to surgical practice in all specialties. Next in the series, we analyze the complex and highly flexible structure that is the knee.

The knee can be tricky business, full of complexities, and challenges. This simulation, available for free in the Touch Surgery™ app, sheds light on the knee from a functional perspective. We take a closer look at why the knee is so prevalently injured, alongside fundamental anatomical knowledge.

This module is presented in four sections: skeletal structure, soft tissue support, muscles and active stabilization, and neurovasculature, providing practical context to the anatomy within and around the knee.

Skeletal Structure

The knee consists of three principal bones: the femur, tibia, and patella, which interact in pairs to form the tibiofemoral and patellofemoral joints. The bone joint structure means the knee is one of the most moveable joints of the body, predominantly moving in flexion and extension.

In this module, you’ll learn about:

  • The screw-home mechanism (key to knee stability)
  • Genu varum and valgum (angular deformities resulting in ‘knock-knees’ or ‘bowlegs’)
  • Q-angle measurement (assesses knee joint function)
  • Total knee arthroplasty (TKA), a common procedure for osteoarthritis
  • Unicompartmental knee arthroplasty (UKA), an alternative procedure to TKA

More information on terminology and surgical intervention is described in the simulation.

The skeletal structure of the knee: the femur, tibia, and patella

Soft Tissue Support

The soft tissue around the knee supports and cushions the bony structure and facilitates normal movement.

Soft tissue support in the knee consists of:

  • Hyaline (articular) cartilage, alongside synovial fluid, allowing the bones to articulate freely with each other.
  • Fibrocartilaginous menisci, which have three purposes:
    • To attenuate the lack of congruence between the femoral and tibial articular surfaces.
    • To increase the contact area of the femur, improving weight distribution.
    • To provide shock absorption for forces during movement.
  • Anterior and posterior cruciate ligaments for anterior-posterior stability during joint movement.
  • Medial and lateral collateral ligaments that act as stabilizers of the knee, attenuating varus and valgus stresses.
  • The capsule surrounding the knee joint, consisting of a fibrous outer sleeve and an inner synovial membrane.

In an anatomical structure as complex as the knee, many body parts have secondary functions in addition to their primary functions. These are depicted in more detail in the module on the Touch Surgery™ app.

Cruciate ligaments of the knee

Muscle and Active Stabilization

Muscles are dynamic stabilizers of the knee, predominantly grouped into extensors and flexors.

  • Quadriceps femoris are the extensors of the knee, consisting of four muscles: rectus femoris, vastus medialis, vastus intermedius, and vastus lateralis.
  • The tensor fascia lata muscle primarily creates static lateral stability but also acts as a weak extensor.
  • The main flexor muscles are the semimembranosus, semitendinosus, and biceps femoris. These muscles are colloquially named the hamstrings.
  • The sartorius and gracilis muscles also aid in flexion and join with the semitendinosus to form the pes anserinus (goose’s foot).
  • The popliteus and gastrocnemius muscles are knee flexors, although they are mainly plantarflexors of the foot.

This module also describes muscular conditions, such as patella maltracking, where a muscular imbalance between vastus medialis and vastus lateralis results in the patella being pulled too much to one side.

Quadriceps femoris or the extensor muscles of the knee

Neurovasculature

Neurovasculature is an important consideration for any surgery of the knee. Failure to recognize compromised neurovasculature can result in dire consequences for the patient and is therefore critical in surgical education.

The main blood supply of the knee is the popliteal artery, which gives rise to four major constant arteries and the middle (posterior) genicular artery.

Each nerve supplies the muscles in the knee. These are explained in detail in the simulation.

Sensory innervation of the knee is divided into four quadrants: superomedial, superolateral, inferomedial, and inferolateral.

Sensory innervation quadrants of the knee: superomedial, superolateral, inferomedial, and inferolateral

If you’re keen to learn more about the challenges and complexities of the knee head over to the Touch Surgery™ app now!

Launch in App

References

1. Wilson HA, Middleton R, Abram SG, Smith S, Alvand A, Jackson WF, Bottomley N, Hopewell S, Price AJ. Patient relevant outcomes of unicompartmental versus total knee replacement: systematic review and meta-analysis. BMJ. 2019;364.

2. Gwinner C, Märdian S, Schwabe P, Schaser KD, Krapohl BD, Jung TM. Current concepts review: Fractures of the patella. GMS Interdiscip Plast Reconstr Surg DGPW. 2016;5.

3. Khadavi M and Fredericson M. ACL Tear: Causes and Risk Factors Website. Veritas Health. https://www.sports-health.com/sports-injuries/knee-injuries/acl-tear-causes-and-risk-factors. Updated June, 12 2020. Accessed on October, 22 2020.

4. Harner CD, Höher J. Evaluation and treatment of posterior cruciate ligament injuries. Am J Sports Med. 1998;26(3):471–482.

5. Chahla J, Moatshe G, Dean CS, LaPrade RF. Posterolateral corner of the knee: current concepts. ABJS. 2016;4(2):97.

6. Tran J, Peng PW, Lam K, et al. Anatomical Study of the Innervation of Anterior Knee Joint Capsule: Implication for Image-Guided Interventions. Reg Anesth Pain Med. 2018;43:407–414.

7. Trammell AP, Pilson H. Anatomy, Bony Pelvis and Lower Limb, Tensor Fasciae Latae Muscle. Treasure Island, FL: StatPearls Publishing; 2018.

8. Anderson MJ, Browning WM 3rd, Urband CE, Kluczynski MA, Bisson LJ. A Systematic Summary of Systematic Reviews on the Topic of the Anterior Cruciate Ligament. Orthop J Sports Med. 2016;4(3).

9. Chen L, Kim PD, Ahmad CS, Levine WN. Medial collateral ligament injuries of the knee: current treatment concepts. Curr Rev Musculoskelet Med. 2008;1(2):108–113.

10. Coyle C, Jagernauth S, Ramachandran M. Tibial eminence fractures in the paediatric population: a systematic review. J Child Orthop. 2014;8(2):149–159.

11. Fox AJ, Bedi A, Rodeo SA. The basic science of human knee menisci: structure, composition, and function. Sports Health. 2012;4(4):340–351.

12. Kannus P, Bergfeld J, Järvinen M, Johnson RJ, Pope M, Renström P, Yasuda K. Injuries to the posterior cruciate ligament of the knee. Sports Med. 1991;12(2):110–131.

13. Logterman SL, Wydra FB, Frank RM. Posterior Cruciate Ligament: Anatomy and Biomechanics. Curr Rev Musculoskelet Med. 2018;11(3):510–514.

14. Makris EA, Hadidi P, Athanasiou KA. The knee meniscus: structure-function, pathophysiology, current repair techniques, and prospects for regeneration. Biomaterials. 2011;32(30):7411–7431.

15. Otsubo H, Shino K, Suzuki D, Kamiya T, Suzuki T, Watanabe K, Fujimiya M, Iwahashi T, Yamashita T. The arrangement and the attachment areas of three ACL bundles. Knee Surg Sports Traumatol Arthrosc. 2012;20(1):127–134.