[MEDIN implant of the first metatarsophalangeal joint]

Hemiarthoplasty or total replacement of the first metatarsophalangeal (MTP) joint has been used in orthopaedic surgery for the last 60 year, but good post-operative outcomes have been achieved only in the last ten years. Joint replacement is mainly used in stage 3 and stage 4 hallux rigidus conditions for which arthrodesis is not indicated. The operation on the first MTP joint has its place in the present-day orthopaedics. This study describes anatomical measurements and the development of the first Czech implant (MEDIN Orthopaedics) to replace this joint.

Thirty cadaver specimens were used to develop basic shapes of phalangeal and metatarsal components. A standard technique was used for anatomical dissection of the first MTP joint. Fifteen specimens were cut in the sagittal plane and fifteen in the transverse plane in order to open the intramedullary cavity of the proximal phalanx of the great toe and the first metatarsal bone. The basic shapes of phalangeal and metatarseal components were designed based on the shape of cortical bone of their inner surfaces. Data for the shape, size and scale of articular components were obtained by measurement on 58 dry bone specimens of the first metatarsus and on 30 calibrated X-ray images. In order to adjust the scale and size of components, the final shape and the range of implant size were tested on 50 specimens of dissected lower extremities fixed in formaldehyde, acetone, ethyl-alcohol and glycerol.

The new type of a first MTP implant designed by us was based on cone-shaped anchor components coated with hydroxyapatite. The implants can be used in hemiarthroplasty or total joint replacement. The metatarsal insert was designed with a declination angle of 20 degrees to facilitate good dorsiflexion and with a flattening to ensure good function of the sesamoid bones, The phalangeal articular insert was made of either CoCr alloy or low-weight polyethylene for use in hemiarthroplasty and total joint replacement, respectively.

The new implants are designed for treatment of stage 3 or stage 4 hallux rigidus. We recommend to use hemiarthroplasty or total joint replacement only in the case of first metatarsal head destruction or severe joint destruction due to rheumatoid arthritis.

Our anatomical study of the first MTP joint, proximal phalanx of the great toe and first metatarsal bone was used to design the first Czech implant of this joint.