Fundamental in nature is that every human being is unique, formed from the union of two cells, growing in utero primarily by genetic pre-programming following the miraculous steps of embryogenesis.

Our skeletons and our entire musculoskeletal systems function according to the principles of mechanobiology. The cells from the middle layer of an embryo, termed the mesenchyme, form the musculoskeletal system, and are imbued with the potential to change their nature and their structure according to their chemical and physical environment, especially applied physical force. Our bodies mirror the activities of our lifetimes.

Our knees form in the embryo between 8 and 20 weeks, starting out as a solid wall of primitive cells that are termed blastemal or mesenchymal, and each has this characteristic ability to change in response to its environment. Many cells simply die through a process called apoptosis – programmed cell death, creating space within the borders of the anterior compartment. Over the course of just a few weeks all elements of the knee form from these pre-cursor cells: the bones, the joint capsule, the menisci, and the main ligaments. In the anterior compartment, the group of formative cells is called a blastema, and its task is to create structural elements that form according to the physical stresses applied as the emerging human being begins to move.

At first there is not much movement in the developing limb of the embryo. With minimal applied force the fibrous tissue first is termed loose connective.  As the actual bony structures, surrounding muscles, capsule, and all the other elements of the knee are formed over a matter of weeks, movement begins and the forces become more robust. Necessary supporting vessels and nerves invade the anterior compartment from the periphery. The fat pad is supplied by every nerve in the limb -- femoral, tibial, common peroneal, recurrent peroneal, and obturator. This embryological fact implies that pain generated from the fat pad can thus be perceived anywhere in and about the knee. The fibrous tissue with more limb motion morphs to becomes a mixture of dense and loose connective tissues and increasingly, lobular fat appears. By 20 weeks the tissues have fully organized and the complete structure that is the fat pad is present, compressible fat filling the highly innervated dense connective tissue mesh.

With increasing motion the attachment of the fat pad to the femur organizes into a linear connective tissue structure, the infrapatellar plica (IPP) or ligamentum mucosum, a mélange of loose connective tissue, dense connective tissue, fat, vessels and sparse nerves. It parallels the anterior cruciate ligament (ACL), and can have diverse architecture, from one or more separate, rope-like bands that are not attached to the anterior cruciate ligament (ACL). Or it can be a solid wall of tissue attached to the ACL, sometimes with an opening or fenestra in it. Anterior Knee Pain (AKP) in association with this type of IPP, in our clinical experience can become highly symptomatic, and requires care so that it is completely removed.

Importantly, the dense connective of the IPP is formed from collagen, the key structural element making it an intra-articular ligament.

Most of the time (86.5% according to Kim’s arthroscopic survey - ref. 1), the knee contains an IPP. This course focuses on the AKP that is associated with those that have an IPP and in whom the physiology of the complex of tissues that is the IPP-fat pad complex is present. Sometimes the fat pad is not attached to the femur at all (14.5% in Kim’s survey). The fat pad is thus not constrained by the central tether of the IPP. The incidence of AKP in such individuals has not been studied.

 

 

Collagens

Complex chemical composites are the main structural elements of our bodies. Those involved in the make-up of the musculoskeletal system are different types of collagens.

The key collagens include:

  • collagen type I, the core element of bones, ligaments, and joint capsules;
  • collagen II, the core element of cartilage; and
  • collagen III, also called reticulin, forming the more delicate supporting sheets of tissue on the margins of the IPP.

I mention this because our collagen make-up is an inherited trait. Knee hyperextension is a factor in the development of AKP and there is a wide range of knee laxity. There are many sub-types of collagen which have varying degrees of laxity. Every person has his or her unique genetic make-up of collagens leading to a spectrum of joint laxity.  In general, those who have “loose joints” will have knee hyperextension, a trait that is associated with an increased incidence of AKP, as in the female athletic population.

 

References

1. Kim SJ, Min BH, Kim HK. Arthroscopic anatomy of the infrapatellar plica. Arthroscopy. 1996 Oct;12(5):561-4.

 

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