Aspiration as an aid to the diagnosis of prosthetic knee instability.

Knee instability is a common cause of prosthetic knee failure. This study sought to determine if synovial fluid analysis is a useful tool in diagnosing prosthetic knee instability. Aspirations were performed in 42 knees in which the diagnosis of instability was in doubt. This group was compared to 120 aspirated knees performed for other diagnoses, excluding infection. The average red blood cell count for the instability group was 79,365 (range, 300-960,000). The median red blood cell count was 9175 (interquartile range, 37,750-2337). In patients with other diagnoses, the average red blood cell count was 60,965 (range, 30-2,079,000). The median red blood cell count was 6200 (interquartile range, 27,525-2512; P=.56). This study confirms that unstable total knees have bloody effusions. The prognostic significance of this finding must be questioned, as patients with other diagnoses have similar findings. Physical examinations and stress radiographs remain the key determinants of prosthetic knee instability.