A comparative study between two stems (Linea Anatomic and Minihip) has been performed in order to analyse the differences in their biomechanical behaviour, concerning stem micromotions and load transmission between stem and bone. From the corresponding finite element models, a parametric study was carried out to quantify ranges of micromotions taking into account: friction coefficient in the stem-bone interface, press-fit and two types of gait cycle. Micromotions were evaluated for each stem at six different levels along repeated gait cycles. An initial and marked stem subsidence at the beginning of the simulation was observed, followed by an asymptotic decrease due to friction forces. Once migration occurs, a repeated reversible cyclic micromotion is developed and stabilized as gait cycle times are simulated. The general motion pattern exhibited higher amplitude of micromotion for Minihip compared to Linea stem. The load transmission mechanism was analyzed, identifying the main internal forces. The results show higher local forces for Minihip stem up to 80% greater than for Linea stem. The differences of design between Minihip and Linea conditioned different distributions of load, influencing the posterior stress-shielding. Consequently, short stems require high bone stock and quality should, being indicated for young patients with high bone quality.

Osteomyelitis is an uncommon infection that is considered to be a childhood disease. Diagnosis is frequently delayed, as it is a very rare condition and also because of its variable presentation. After an accurate diagnosis and treatment, the outcome is favorable in children. We present one case without recent previous infection, antecedent trauma, or penetrating injury, illustrating the difficulties in diagnosis. Nontraumatic osteomyelitis of the patella should be regarded as a rare hematogenous infection. A high index of suspicion should be addressed for early recognition. Study design is level of evidence: level IV.