In order to extend the lifetime of total knee replacements (TKR) in vivo, reduction of the volumetric wear rate of ultra high molecular weight polyethylene (UHMWPE) bearings remains an important goal. The volume of wear debris generated in fixed bearing total knee devices increases significantly when subjected to higher levels of internal-external rotation and anterior-posterior displacement. Six PFC Sigma fixed bearing TKR were compared with six LCS rotating platform mobile bearing knees using a physiological knee simulator with high rotation kinematic inputs. The rotating platform polyethylene inserts exhibited a mean wear rate which was one-third of that of the fixed bearing inserts despite having increased femoral contact areas and additional tibial wear surfaces. The rotating platform design decouples knee motions, by allowing unidirectional motion at the tray-insert articulation, which reduces rotation at the femoral-insert counterface. This translation of complex knee motions into more unidirectional motions results in molecular orientation of the UHMWPE and reduced volumetric wear.

Journal Article↗

PubMedDate not supplied

Comparison of wear in a total knee replacement under different kinematic conditions.

A six station ProSim (Manchester, UK) knee simulator was used to assess the wear of six PFC (DePuy) fixed bearing total knee replacements under two different kinematic conditions defined as low and high kinematic inputs. The high kinematics displacement and rotation inputs were based on the kinematics of the natural knee with ISO standards used for the axial load and flexion. Low kinematics were defined as approximately half the magnitude. The six specimens were run for three million cycles under low kinematics and three million cycles under high kinematics. The mean wear rate found during the low kinematics phase was 7.7 +/- 2 mm3 per million cycles. This then increased significantly to an average wear rate of 41 +/- 14 mm3 during the high kinematics input phase. The wear areas were characterized by a predominant damage mode of burnishing with some abrasive wear occurring during the high kinematics phase. This study supports the findings that introduction of cross-shearing of the polyethylene by introducing both rotational and anterior/posterior displacement increases the wear rate. This has implications for younger patients with higher levels of activity that need knee replacements.

Journal Article↗

PubMedDate not supplied

Analytically oriented psychotherapy in schizotypal and borderline patients: at the border of treatability.

Analytically oriented psychotherapy (AOP) has been considered the treatment of choice for borderline patients and a useful technique in the treatment of schizotypal patients. There are many exceptions, however, in addition to a number of borderline and schizotypal patients who are just barely amenable to this modality: they are at the border of treatability by AOP. Limitations relating to time, cost, and the availability of therapists trained in this discipline render it important to delineate the factors which conduce either to the success or failure of AOP. From the author's clinical impressions about borderline and schizotypal patients at the border of treatability by AOP, a number of such factors emerge. On the positive side: likeableness, autoplastic defenses, high motivation, psychological-mindedness, genuine concern, good moral sense, self-discipline, and low impulsivity. Negative factors include, beside the opposites to the aforementioned, vengefulness and parental abusiveness or exploitation. A scale for measuring the balance between these positive and negative factors is proposed. Its use may, it is hoped, improve forecast, during initial consultation, as to which borderline and schizotypal patients will respond favorably to AOP.

Adult↗