Background: Aseptic loosening and osteolysis
are rarely associated with cemented posterior-cruciate-substituting total
knee replacements. Consequently, there is a paucity of information
on this topic.
Methods: After a mean follow-up interval of fifty-six
months (range, thirty-seven to eighty-nine months), sixteen (2.9%)
of 557 posterior-cruciate-substituting primary total knee replacements
were revised by a single surgeon because of loosening and osteolysis. Clinical,
radiographic, and retrieval analyses were conducted to determine
the mechanism of loosening and to identify associated risk factors.
Results: All sixteen knees (fifteen patients) were
rated as good or excellent at one year after the primary replacement,
with mean clinical and functional Knee Society scores of 95 and
86 points, respectively. Nine of the fifteen patients who had a
revision because of loosening and osteolysis had had a total knee
arthroplasty on the contralateral side compared with only 18% of
the patients who did not have a revision (p = 0.026). No
evidence of transmission of substantial anteroposterior stresses from
the posterior-cruciate-substituting mechanism was found. All twelve
retrieved knee implants, however, had damage to the lateral and medial
side walls of the polyethylene posterior-cruciate-substituting post.
Damage to the inferior surface of the polyethylene inserts had a
rotational pattern, with the axis of rotation in the medial compartment.
Surface damage in a rotational pattern was also present on the superior
and inferior surfaces of the titanium tibial base-plates.
Conclusions: In the knees in our study, rotational
forces were generated by impingement of the side walls of the intercondylar
box on the polyethylene post. Such box-post impingement can occur
throughout the range of motion. Rotational stresses are transmitted to
the modular interfaces and to the metal-cement interfaces, resulting
in loosening and osteolysis. A reduction in rotational constraint
would be desirable. Patients with bilateral total knee replacement may
be at increased risk for this type of loosening.