Facet joint innervation is much more than just the medial branches.
In a manuscript by Mapp and Walsh titled : Mechanisms and targets of angiogenesis and nerve growth in osteoarthritis , they reviewed articles that addressed angiogenesis and nerve ingrowth of the synovial joint including the osteochondral environment in the paradigm of osteoarthritis.
These researchers looked at the anatomical environment of the osteochondral junction as well as the meniscal tissue. They suggested it was a difficult task to determine what ‘normal’ is when factoring in the age-related factors. Angiogenesis is a normal part of development but it is also pathological when looking at cancer and osteoarthritis mechanisms. Angiogenesis cultivates nerve ingrowth as neurovascular bundles travel together. Most pain from the facet is thought to come from the medial branches but there are also nerves from the subchondral area that are similar to the basivertebral nerve innervating the endplate. As cartilage is damaged, underlying subchondral bone can be exposed where the tide mark exists and it is thought to drive this angiogenesis and nerve ingrowth.
The knee is the most commonly studied synovial joint. Interestingly, the meniscus (a fibrocartilage) is thought to be only innervated on the outer third. For those that have studied the intervertebral discs, this should sound familiar. We know in the spine, the outer third of the disc is innervated. It is also fibrocartilagenous with very little blood vessels. In this publication by Mapp and Walsh they talked about when the meniscus is damaged, nerve ingrowth into tissue that is normally not innervated, becomes innervated and sensitive. We know this also occurs in the intervertebral discs and modeled in the Professional LxH Model.
Facet joint innervation is therefore found to exist not only with the capsular tissues that engulf the synovial joint but also deep within the joint itself. At the osteochondral junction nerves grow from the subchondral bone and into the underlying osteochondral areas. Furthermore, osteophytes (bone spurs) have been shown only to occur on the periphery of synovial joints. The exact mechanism of their development is not known. Some theorists believe they are a result of chondrocyte proliferation as they look like epiphyseal growth plates.
One common theme that resonates across all joint pathologies is the reduction of joint space width. Therapeutic strategies to increase joint space should always be explored when looking at ways to reduce pain.
Dynamic Disc Designs Corp. strives to provide a dynamic look at pain generators in a model platform that professionals can feel confident in using when explaining pain syndromes to their patients.