Abstract
Introduction:
Many variables contribute to aseptic loosening, and the release of wear particles is a predominant source of late failure. It has been difficult to measure TKA wear quantitatively from retrieved devices; hence, there is a relative paucity of clinically observed TKA wear rates in the literature. Additionally, little is known about patient factors influencing wear rates. This study (a) establishes a clinically relevant TKA wear rate for a cruciate retaining TKA design and (b) relate those wear readings to gait measures of their hosts.
Methods:
34 revision- and 11 postmortem-retrieved MG II tibial PE-components were included in the analysis. Wear scars on the articulating surface of the insert were digitized under light microscopy. The geometry of the surfaces was mapped at 100×100 μm using a low-incidence laser. Autonomous mathematical reconstruction of the original surface was used [1], and linear penetration on the medial and lateral surfaces and total wear volume were calculated (Fig-1).
For five implants, gait data recorded during 1.5 years after surgery were available. Gait studies were performed using a three-dimensional optoelectronic system for motion capture. Joint kinematics and kinetics were calculated using a six-marker model of the lower extremity [2]. All knee moments are reported in Nm, acting externally at the tibia. Potential linear relationships between wear and moment characteristics were investigated.
Results:
Total wear correlated linearly with time in situ (R2 = 0.42) resulting in a volumetric wear rate of 14.3 ± 2.8 mm3/year (Fig-2). The five tibial inserts with gait information came from two female and two male patients (one of them bi-lateral). Their time in situ ranged from 16 to 98 months and their total volume loss from 11.3 mm3 to 258.2 mm3. At the time of testing, the patients were 55 ± 9 years old, pain free and walked at speeds of 1.1 ± 0.3 m/s. Except for one, subjects walked with slight flexion contracture at heel strike and reduced mid-stance flexion. Subjects further showed diminished flexion moments during mid-stance, an observation linked to ‘quadriceps reduced’ gait. Lower flexion moments were associated with a lower wear rate (R2 = 0.60). The correlation improved (R2 = 0.97) when the component being the least time in situ was removed from the analysis. Its wear rate of 83.1 mm3/yr, a running-in phenomenon, was significantly larger than the 19.8 mm3/yr average of the four remaining components. Also, the adduction (‘varus’) moment was positively associated with wear rate (R2 = 0.79). However, this association was largely driven by the implant with the high wear rate. Its removal dropped the correlation factor to 0.16. No relationship was found for internal and external rotation moments.
Discussion:
As expected, wear of tibial PE-inserts increased with time in situ. To our knowledge, this is the first study reporting a reliable TKR clinical wear rate. Comparison to gait data suggests there are additional patient factors, other than ‘use’, that govern wear rate. Particularly external knee joint moments are associated with wear, a finding which underscores the role of joint moments as surrogate markers for contact load.
