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.
Knee instability following total knee arthroplasty is a common cause of early prosthetic knee failure. In a study of 440 patients referred for revision surgery to our institution, 279 had early failure requiring revision within 5 years of index arthroplasty. Seventy-four of those patients (27%) had revision surgery because of prosthetic knee instability.1 Instability also has been reported in several previous studies as a common cause of failure and reason for revision in 10% to 22% of revised knees.2-7
The diagnosis of prosthetic knee instability is frequently elusive. Clinical presentation can take many forms ranging from obvious dislocation to vague reports of diffuse anterior knee pain, giving way, or recurrent effusion. In cases of gross instability, the diagnosis is seldom in doubt. However, in patients who have subtle instability patterns, guarding in the examination room frequently precludes an accurate examination and subsequent diagnosis.
This group of patients frequently presents with normal-appearing radiographs and multiple subjective symptoms. In order to facilitate an accurate diagnosis, a reliable diagnostic test would be helpful to distinguish patients with subtle instability from those with merely a dystrophic pain pattern.
A previous study from our institution noted a predominance of red blood cells from aspirates of unstable total knees.8 While knee aspiration and synovial fluid analysis has been helpful in making the diagnosis of infection,9 the usefulness of synovial fluid analysis for other diagnoses has not been studied.1,10,11 The purpose of this study was to determine if synovial fluid analysis can be a useful diagnostic tool in making the diagnosis of prosthetic knee instability.
Materials and Methods
Between 1987 and 2006, 133 total knees were revised for instability at a single institution. Aspirations were performed selectively in this group of patients as part of a preoperative protocol. Aspirations were performed when the preoperative serologies (C-reactive protein and sedimentation rate) were elevated or when the diagnosis of instability was in doubt. Patients with gross instability and normal serology were not aspirated. Medical records including hospital laboratory reports and clinic charts were retrospectively reviewed. When aspirates were performed, red blood cell counts were documented.
Forty-two of the 133 knees (40 patients) revised for instability were aspirated prior to revision surgery. Of the 40 patients, 20 were women and 20 were men. Average age at the time of revision surgery was 61.8 years (range, 40-86 years). The aspirates of this group of 42 knees revised for instability were compared to a control group of 120 knees (114 patients) revised during the same time period for reasons other than instability or infection. Of these 120 revision knee cases (114 patients), 50 were for wear or lysis, 45 were for aseptic loosening, and 25 were for miscellaneous reasons, including extensor mechanism/patella problems (n=6), flexion contracture/arthrofibrosis (n=6), periprosthetic fracture (n=4), malalignment/malposition (n=4), implant fracture (n=4) and failed unicondylar arthroplasties (n=1). There were 68 women and 46 men. Average age at the time of revision surgery was 64.8 years (range, 34-86 years).
Standard descriptive statistics were calculated, including mean, median, interquartile range, range, and frequency. Statistical differences between groups were determined using a Student t test for normally distributed parametric data. A Mann-Whitney U test was used to determine statistical differences between groups for nonparametric data or parametric data that was not normally distributed.
The average red blood cell count for patients revised for a diagnosis of instability was 79,365 red cells (SD 195,738; range, 300-960,000) per cubic millimeter. The median red blood cell count was 9175 red blood cells (interquartile range, 37,750-2337) per cubic millimeter.
In patients with diagnoses other than instability and infection who were revised and aspirated, the average red blood cell count in the synovial fluid was 60,965 red cells (SD 230,074; range, 30-2,079,000) per cubic millimeter. The median red blood cell count was 6200 (interquartile range, 27,525-2512). This difference between the average red blood cell count for the group of patients with instability and the group with diagnoses other than instability and infection was not statistically significant (P=.56), with the numbers available.
Knee instability is increasingly recognized as a mode of prosthetic knee failure. Such instability is frequently technique related. Failure to adequately release and balance contracted ligaments, as well as failure to equalize flexion and extension gaps, are common errors leading to prosthetic knee instability. Gross malalignment and unrecognized ligamentous incompetence not compensated by prosthetic constraint are other causes of instability directly under the surgeons control.
Patients with prosthetic knee instability present in various ways, ranging from frank dislocation to vague reports of pain with normal radiographs. A careful history and physical examination as well as stress radiographs are the primary tools available to a clinician in making a diagnosis. In a patient with tibial femoral dislocation or a grossly incompetent medial collateral ligament, the diagnosis is rarely in doubt. However, in a patient with normal-appearing radiographs and excellent range of motion, the diagnosis of instability can be difficult.
Subtle forms of instability caused by flexionextension mismatch, posterior cruciate insufficiency, or unbalanced collateral ligaments can be recognized through a careful history. Patients usually express their dissatisfaction by saying that their knee gives way or wont hold them up. Some patients report that they have persistent anterior knee pain or that they have to just get their knee in gear before ambulating after being seated. These symptoms combined with a careful physical examination stressing the knee in both the anteroposterior and the medial lateral plane at varying flexion angles usually helps make the diagnosis.
Other than stress radiographs, objective diagnostic testing to assist the clinician in making the diagnosis of prosthetic knee instability is lacking. In our original series describing 25 cases of prosthetic knee instability, we noted a predominance of red blood cells on the aspirates of patients revised for instability.7 However, preoperative cell counts from the aspirates were available in only half of the patients in this series and no comparative group was analyzed. Therefore, we embarked on this study to determine if a predominance of red blood cells on aspirate from patients with suspected prosthetic knee instability could be used as a diagnostic indicator.
We hypothesized that a preoperative aspirate and careful review of the cell count could give us similar information as our previous study documenting the usefulness of aspirated white blood cell counts in making the diagnosis of infection.9 In this study we found that a white blood cell count of >2500 with 60% polymorphonuclear cells was highly suggestive of infection. Unfortunately, we were unable to make such a distinction regarding the presence of hemarthrosis and a specific diagnosis of instability. We had originally theorized that a predominance of red blood cells on aspirates was related to microtrauma of the synovial lining and was specific for the diagnosis of instability.
The purpose of this study was to determine if synovial fluid analysis could be a useful diagnostic tool in making the diagnosis of prosthetic knee instability. While we have shown that unstable total knees have preoperative bloody effusions with a median red blood cell count of 9175, patients revised for other diagnoses frequently present with similar results when aspirated (median red blood cell count, 6200).
One of the limitations of this retrospective review is that not all of the patients revised during this time period for instability were aspirated. Selective aspirates were the norm, and patients with gross instability who were not a diagnostic dilemma were not aspirated. A well-designed prospective study that includes aspiration of all patients revised for instability may have made a difference in the findings reported here. Another limitation is that there was no documentation of a traumatic aspiration. This is usually manifested by an initially clear aspirate followed by a small amount of blood.
Regardless of these limitations, our study confirms that unstable total knees have preoperative bloody effusions. This is thought to be related to the microtrauma of the synovial lining due to a specific instability pattern. The prognostic significance of this finding, however, must be questioned as patients with other diagnosis have a similar number of red blood cells in synovial aspirates.
While hemarthrosis is frequently present in an unstable knee, it is not pathognomic of prosthetic knee instability. Other diagnoses such as aseptic loosening and wear-related problems should also be considered when a predominance of red cells is encountered on aspirate. Therefore, a careful history and physical examination as well as stress radiographs remain the key determinants of prosthetic knee instability. A bloody aspirate can help confirm the diagnosis of instability suspected on history and physical examination.
- Fehring TK, Odum S, Griffin WL, Mason JB, Nadaud M. Early failures in total knee arthroplasty. Clin Orthop Relat Res. 2001; (392):315-318.
- Friedman RJ, Hirst P, Poss R, Kelley K, Sledge CB. Results of revision total knee arthroplasty performed for aseptic loosening. Clin Orthop Relat Res. 1990; (255):235-241.
- Goldberg VM, Figgie MP, Figgie HE III, Sobel M. The results of revision total knee arthroplasty. Clin Orthop Relat Res. 1988; (266):86-92.
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- Jacobs MA, Hungerford DS, Krackow KA, Lennox DW. Revision total knee arthroplasty for aseptic failure. Clin Orthop Relat Res. 1988; (226):78-85.
- Rand JA, Bryan RS. Revision after total knee arthroplasty. Orthop Clin North Am. 1982; 13(1):201-212.
- Thornhill TS, Dalziel RW, Sledge CB. Alternatives to arthrodesis for the failed total knee arthroplasty. Clin Orthop Relat Res. 1982; (170):131-140.
- Fehring TK, Valadie AJ. Knee instability after total knee arthroplasty. Clin Orthop Relat Res. 1994; (299):157-163.
- Mason JB, Fehring TK, Odum SM, Griffin WL, Nussman DS. The value of white blood cell counts before revision total knee arthroplasty. J Arthroplasty. 2003; 18(8):1038-1043.
- Duff GP, Lachiewicz PF, Kelley SS. Aspiration of the knee joint before revision arthroplasty. Clin Orthop Relat Res. 1996; (331):132-139.
- Barrack RL, Jennings RW, Wolfe MW, Bertot AJ. The Coventry Award. The value of preoperative aspiration before total knee revision. Clin Orthop Relat Res. 1997; (345):8-16.
Dr Raab is from the Bone and Joint Institute, Penn State Hershey Medical Center, Hershey, Pennsylvania; and Drs Fehring, Mason, and Griffin and Ms Odum are from OrthoCarolina, Charlotte, North Carolina.
Dr Raab has no relevant financial relationships to disclose. Drs Fehring, Griffin, and Mason are consultants for and receive research support and royalties from DePuy. Ms Odum receives research support from DePuy.
Correspondence should be addressed to: Thomas K. Fehring, MD, OrthoCarolina, 1915 Randolph Rd, Charlotte, NC 28207.