1st Metatarsophalangeal Joint Osteoarthritis

After more than 30 years of clinical practice using Biomechanical knowledge, skills and insights into effective orthotic prescription techniques, Ray Harding of Harding Podiatry, will share with you the reason why the practice has an unmatched reputation in this field.

What is it, how does it occur and how can this condition be treated?

This is a fairly common condition in which the patient presents with pain and inflammation in an enlarged 1st MTP joint. This pain occurs in walking and running activities, specifically during the propulsive phase of gait. The patient may also complain of dull, aching pain at night when there is significant inflammation.

Weight bearing Xrays usually show an enlarged metatarsal head and proximal phalanx of the toe, with loss of joint space, irregular rather than smooth, curved and parallel joint surfaces and possible ‘pac-man’ shaped erosions and bone cysts. There are often osteophytes on the joint margins, most often on the dorsal surface of the metatarsal. The sesamoids may also be misplaced from their normal position, most commonly in a lateral direction.

On palpation the joint is enlarged and irregular in shape and may be painful if compressed and manipulated. In most cases the range of both dorsiflexion and plantarflexion is reduced with movement in dorsiflexion causing the most discomfort. Pain when walking or running is from the joint functioning at the end range of dorsiflexion.

The cause of this condition will be evident following a thorough biomechanical examination of lower limb alignment and functional position. Any pathology, such as an internally rotated hip position, internal tibial torsion, genu valgum, ankle equinus, etc. that is compensated by excessive subtalar and midtarsal joint pronation will increase load on the medial aspect of the foot and reduce the medial longitudinal arch contour. In a static weight bearing position the flexor hallucis longus tendon and the medial band of the plantar fascia will be tightened, plantarflexing the hallux and creating a functional hallux limitus.

During gait, if this compensation causes the foot to remain pronated throughout the midstance phase of gait into propulsion, there will be abnormal increased load through the 1ST MTPJ and restricted available dorsiflexion. An abductory twist at heel off, to propel around this restricted propulsive lever, places shearing stress on the already overloaded joint.

With normal alignment and function, the subtalar joint resupinates prior to the propulsive phase of gait, locking the midtarsal joint and stabilising the forefoot through MTJ pronation. This allows the tendon of peroneus longus to stabilise the 1ST metatarsal in a plantar flexed position, creating an effective propulsive lever, no restriction to the range of dorsiflexion and no tightening of FHL tendon or plantar fascia.

This condition can also occur with a high arched foot type created by a forefoot valgus alignment and/or a plantar flexed 1ST ray. In this foot type the forefoot is everted in relation to the rearfoot when the subtalar joint is in a neutral position and the midtarsal joint is maximally pronated. This would normally be a perpendicular relationship.

On weight bearing, ground reaction force inverts the everted forefoot through midtarsal joint supination. This unlocks the forefoot making it unstable during gait as well as dorsiflexing the 1ST metatarsal. This creates a functional hallux limitus and overloads the 1ST MTPJ during propulsion. If more than eight degrees of forefoot valgus compensation is required then the subtalar joint also supinates, inverting the rearfoot and creating lateral ankle joint instability.

If the ‘Foot Posture Index’ is used for assessing the degree of pronation or supination in static stance and any need for orthotic therapy, then a high arched foot type with a forefoot valgus or plantar flexed 1ST ray that is compensated by subtalar joint supination but an unlocked, supinated midtarsal joint would give a false impression of stability.

Conservative treatment involves the use of NSAIDs to reduce pain and inflammation as well as orthotic therapy and appropriate footwear. Prior to dispensing orthoses, a shoe whose midsole restricts toe dorsiflexion can provide some relief. The thorough biomechanical examination will provide the Podiatrist with the prescription for orthoses incorporating adequate rearfoot control with rearfoot posts incorporating a bevel for adequate pronation at heel strike through early midstance. A forefoot post is required for forefoot valgus plus a 2-5 bar or metatarsal dome to offload a plantar flexed 1ST ray. This is necessary to maintain a stable forefoot during the propulsive phase of gait with a locked midtarsal joint. This should reduce load on the 1ST MTPJ and allow pain free dorsiflexion.

For precise orthotic prescription and orthotic shell contour it is critically important to capture the shape of the foot and forefoot to rearfoot relationship in its ideal alignment using either a plaster cast taken with suspension casting technique or 3D scan where the shape and forefoot alignment can be measured and compared to the biomechanical examination.

One of the contributing causes of this condition is torsional flex within the foot and shoe. Therefore, recommendation of an appropriate shoe requires restriction of flexibility in the sagittal plane and restriction of torsional flexibility on the frontal plane. Likewise, the material chosen for the prescription orthoses must also be able to withstand torsion flexibility. Therefore, a rigid or semi-rigid orthotic shell material is preferred to a flexible material like EVA.

Having said that, I often use high density EVA for full length orthoses with an extended forefoot valgus when prescribing for athletes that will wear the orthoses in running shoes and football boots. These provide adequate midtarsal joint control and therefore ability to offload the 1ST MTPJ during propulsion. Soft, cushioning and flexible materials are useless for the orthotic shells but are invaluable when providing cushioning directly plantar to the painful joints.

Sometimes when the 1ST MTPJ is offloaded and free dorsiflexion is then available, pain can still be created by the joint functioning at the end range of motion due to the deformity in bone shape. In sever cases like these an extension to the orthotic shell, such a ‘Morton’s extension’ can prevent joint dorsiflexion, enabling pain-free mobility. Unfortunately, this creates an intoed foot position during propulsion.

Although the pain and inflammation in and around the joint may reduce with less stress and loading, the bone shape and deformity will remain. The only remedy is the surgical removal of osteophytes to increase range of motion or fusion of the joint to prevent all painful movement.

Unfortunately, there are few positives with OA of the 1ST MTPJ as the condition cannot be reversed. However, the conservative orthotic therapy approach does at least address the cause of the condition whereas surgery could make the overall condition and functional position worse. We have been able to get many patients back to pain free activity with functional orthoses, including professional athletes.

If you need advice on prescription orthotics, book an appointment with one of our podiatrists for an assessment.

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