DJD Enhanced Forefoot Rocker

Osteoarthritic changes are known as degenerative joint disease (DJD). It is the most common cause of 1st metatarsophalangeal (MTP) joint dysfunction and ‘big toe’ pain. Loss of free hallux motion in gait, particularly during the acceleration phase, is problematic for gait efficiency and often links to symptoms elsewhere. HealthyStep’s famous X-Line DJD has revolutionised the treatment of these issues.

Our clinically experienced design team acknowledged that sometimes increasing the forefoot rocker element of the DJD was going to be necessary for some patients. Therefore, an addition that can enhance the acceleration abilities of those afflicted with more extensive 1st MTP joint DJD has been created.

This addition should be considered if the standard DJD improves but does not fully resolve symptoms, particularly during longer walking like hiking. Using the standard DJD in normal shoes and a DJD with the DJD forefoot rocker addition as an enhancement for hiking boots can be the winning combination. Also, consider using a DJD with this addition added, in rocker-soled footwear for the most severe 1st MTP DJD cases.

The DJD enhanced forefoot rocker can also be added to most of our orthotic range so that other conditions can be treated at the same time. 

For example, adding a DJD enhanced forefoot rocker to an X-Line AT can treat both an Achilles tendon issue and concurrent pain from 1 MTP joint DJD.

Mechanical efficiency of gait is greatly enhanced when heel lift occurs in unison with extension at the medial MTP joints, aligned to the hip’s, knee’s and ankle’s axes of rotation. In this situation, the power released from the Achilles elastic recoil at heel lift is coupled to motions of medial digital extension, ankle plantar flexion, knee flexion, and hip extension. Add in some midfoot plantarflexion from tibialis posterior and peroneus longus, and acceleration power is maximised for anterior momentum onto the next footstep. The foot and ankle can work like a linear spring when aligned thus.

The 1st MTP joint with its large metatarsal head should play a prominent part in producing gait’s mechanical efficiency. The ability to drive over rotating medial MTP joints has been termed high gear propulsion. Driving over the lateral MTP is termed low gear propulsion. Low gear heel lift is inefficient for gait as the power exerted via external Achilles moment arm (AEMA) is shortened and often not aligned to the body’s line of progression (A in image below). Compared to high gear (B), low gear acceleration loads the less stable and shorter lateral metatarsals and often causes the rearfoot to invert during acceleration.

If osteoarthritis prevents sufficient hallux extension, low gear propulsion is the most likely compensation. This compensation can resolve the big toe symptoms, but it causes gait mechanics to deteriorate.

Stability of the 1st metatarsal is far more reliant on muscle function than the 2nd or 3rd MTP joints, which are have more stable osseous and articular morphologies. The more mobile 1st metatarsal requires muscular power from flexor digitorum longus and brevis, the oblique head of adductor hallucis, abductor hallucis, and peroneus longus. These muscles are critical to setting an appropriate 1st metatarsal declination angle to allow the hallux to rotate freely on the metatarsal head as the ankle plantarflexes with heel lift. 

There can be problems if either the declination angle is too high or too low. If muscles recurrently fail to create a safe declination angles, the articular cartilage can become damaged causing catastrophic failure in joint biomechanics that leads to subchondral bone sclerosis and cartilage degeneration, known as osteoarthritis. This causes loss of normal MTP joint motion that sets up a cycle of increasing degeneration.

Therefore, the declination angle must lie within a ‘Goldilocks’ zone to permit the safe transfer of Achilles tendon power into motions of heel lifting and medial toe rotation.

To manage the consequences and slow or prevent further 1st MTP joint degeneration, both ankle and MTP joint motions must change but without making walking harder. Stopping 1st MTP joint motion tends to reduce pain but causes a significant loss in normal gait mechanical efficiency (energetics). Morton’s extension pads limit 1st metatarsal plantarflexion and blocks hallux motion to reduce pain, but this also prevents normal high gear propulsion, which is bad!

The X-Line DJD is designed to reduce the ankle dorsiflexion range necessary for heel lift and help set safer metatarsal declination angles. A built-in forefoot rocker helps reduce the range of digital extension necessary during heel lift and acceleration onto the next step. The hallux trench reduces the forces under the hallux trying to extend (dorsiflex) the digit. The trench also allows the hallux to start its motion from a less extended position. This reduces 1st MTP stresses on the diseased joint.

Use of the DJD enhanced forefoot rocker further decreases the amount of MTP joint extension required during acceleration off the forefoot and concurrently lowers the dorsiflexion stresses on the hallux.

With care, DJD enhanced forefoot rockers are suitable for fitting under the shells of Vectorthotic and Alleviate Select span devices when top covers are fitted to them. 

In addition, some of our colleagues have informed us that they are using the DJD for patients with ankle osteoarthritis with great success. A great combination in ankle DJD associated with haemophilia is to use both the heel rocker and the DJD enhanced rocker together on a X-Line DJD or AT insole. The DJD enhanced forefoot rocker also encourages forefoot eversion and adduction during late midstance, better positioning the forefoot for heel lift. This is an added bonus in those with seriously limited ankle motion.