[Total Replacement of the First Metatarsophalangeal Joint by Medin PH-Flex and Its Effect on Forefoot Biomechanics in the Propulsion Phase of the Gait Cycle].

PURPOSE OF THE STUDY Total arthroplasty of the first metatarsophalangeal joint is one of the surgical treatment options for patients with advanced hallux rigidus. This study evaluates the pressure changes in the propulsion phase of the gait cycle using dynamic pedobarography after the total arthroplasty of the first metatarsophalangeal joint by Medin PH-Flex and their comparison with the control groups of patients with hallux rigidus and with asymptomatic patients. MATERIAL AND METHODS Dynamic pedobarograph was used to evaluate 15 first MTP joint replacements by Medin PH-Flex implants in 12 female patients, the average time since joint replacement was 3.5 years (1.5 to 5.5 years). The control group consisted of 13 forefeet in 12 patients with hallux rigidus of Grade 3 and Grade 4 according to the Coughlin classification and 17 healthy patients with no clear foot deformity, i.e. of a total of 34 forefeet. A zone for each metatarsal (M1-M5) and the big toe area (T1) were defined with the use of an integrated software. The difference between the pressure under the big toe area and the first metatarsal bone - hallux stiffness (T1-M1) and the forefoot balance parameter, i.e. the difference between the pressures under the medial and lateral half of the forefoot ((M1+M2)-(M3+M4+M5)), was obtained. The parameters were evaluated for each group during the propulsion phase of the gait, i.e. from 55% to 100% of the stance phase and from 75% to 100% of the stance phase, i.e. that part of propulsion when the greatest pressure is exerted on the big toe. RESULTS For the HS parameter (T1-M1) in 55% to 100% of the stance phase, the median value was -0.66 ± 1.22 (-1.90 to 1.45) in the control group, -0.85 ± 1.94 (-1.40 to 3.80) in patients with hallux rigidus, and -0.10 ± 1.48 (-1.30 to 2.40) in patients after the first MTP joint replacement. The median forefoot balance parameter from 55% of the stance phase was -3.48 ± 2.45 (-6.90 to 0.68) in healthy patients and -4.43 ± 2.72 (-6.98 to 0.23) in hallux rigidus patients. In patients after the joint replacement, the value was -3.00 ± 2.46 (-6.20 to 0.40). The data were statistically analysed by the Dunnett's and Tukey's multiple comparison tests. The hallux stiffness parameter showed a significant improvement after the joint replacement compared to patients with hallux rigidus (p<0.0001). No statistical significance was confirmed when comparing the joint replacement cases and the healthy patients from the control group (p=0.0007 and p=0.0010, respectively). As concerns the forefoot balance parameter, a significant difference was reported in patients with joint replacement compared to healthy patients from the control group and patients with hallux rigidus (p <0.0001). DISCUSSION The published pedobarographic studies differ in terms of the methodology used, the patient population and the parameters examined. The pedobarographic studies after the replacement of the first MTP joint or after its arthrodesis present inconclusive outcomes. According to the available literature, the joint replacement has the potential to improve mediolateral forefoot loading and to partially restore the weight-bearing function of the first ray. Our analysis of the HS parameter suggests that the MTP joint replacement can improve the big toe function compared to patients with hallux rigidus but fails to achieve the functional outcomes of healthy patients. When evaluating the forefoot balance (FB) parameter, we can observe less loading on the lateral half of the forefoot in the propulsion phase compared to hallux rigidus. Nonetheless, the joint replacement is unable to restore the physiological loading of the foot. CONCLUSIONS The first MTP joint replacement has the potential to improve forefoot function and to bring it closer to that of a healthy person, even though achieving physiological loading of the forefoot is unrealistic. Additional studies will be needed to confirm that the indication for the first MTP joint replacement is justified in hallux rigidus in terms of the effect on forefoot biomechanics. Key words: hallux rigidus, total replacement of the first MTP joint, dynamic pedobarography, footscan.