Low rate of infected knee replacements in a nationwide series—is it an underestimate?

Figures & data

Figure 1. Flow chart showing the index operations, endpoint events, and the corresponding data sources.

Figure 2. Flow chart indicating how the data from the Finnish Arthroplasty Register (FAR) and the Hospital Discharge Register (HDR) were combined to create the final dataset. ^aoperation types according to FAR.

Table 1. Patient demographics, perioperative details, and follow-up data for the 38,676 primary unicondylar arthoplasties (UKAs), total knee arthoplasties (TKAs), and revision total knee arthroplasties

	n or median	% or (range)
Age, years	71	(14–96)
Knees in female patients	28,043	73
Diagnosis
Osteoarthritis	34,950	90
Rheumatoid arthritis	3,003	8
Other	723	2
Same-day bilateral arthroplasty	2,854	7
Cement fixation	35,516	92
Patellar resurfacing	12,150	32
Intravenous antibiotic prophylaxis	37,991	98
Antibiotic-impregnated cement	31,873	82
Any early complication	439	1.1
Operating hospital
University hospital	8,405	22
Central hospital	12,999	33
District hospital	12,693	33
Other hospital	4,579	12
Hospitalization ^a
Arrival at hospital on the day of operation	4,185	11
Arrived from home	35,850	97
Length of stay in operating hospital, days	9	(1–106)
Discharged home	25,077	68
Referred for treatment from
Primary healthcare	14,848	38
Private healthcare	10,192	26
Specialized healthcare	8,907	23
Follow-up
Length of follow-up, months	41	(0–103)
Died during the follow-up	3,948	10

^a Data on perioperative hospitalization were available in 36,916 cases.

Table 2. The changes in selected demographic and administrative variables over the observation period (1997–2003)

	Year of operation		p for
	1997	2003	trend
Number of operations	4,514	7,552	–
Type of operation
primary UKA, %	1	5	< 0.001
primary TKA, %	93	90.0	< 0.001
revision TKA, %	6	5	< 0.001
Same-day bilateral procedures, %	5	8	< 0.001
Age at operation, average	71	71	< 0.001
age range	19–93	21–91
Indication for primary TKA
osteoarthritis, %	89	92	< 0.001
rheumatoid arthritis, %	10	6	< 0.001
Indication for revision TKA
loosening, %	34	22	< 0.001
other reasons, %	24	24	0.6
infection, n	29	67	–
infection, %	11	20	0.09
Intravenous antibiotic prophylaxis, %	97	99	< 0.001
Antibiotic-impregnated bone cement, %	58	97	< 0.001
Days of hospitalization, median	10	7	< 0.001
range	1–106	1–48
Arrival on the day of operation, %	2	20	< 0.001
Discharged directly home, %	69	63	< 0.001

UKA: unicondylar knee arthroplasty; TKA: total knee arthroplasty.

Table 3. The endpoint events of follow-up after 38,676 knee replacements. The endpoint events were traced from the records of the Finnish Arthroplasty Register (FAR), the Hospital Discharge Register (HDR), and by combining the data from these two registers (combined endpoint data). If two or more endpoint events were detected for an index operation, only the one occurring first was taken into account in the combined endpoint data

First endpoint event	In combined endpoint data	In FAR data		In HDR data
	n	n	% ^a	n	% ^a
No endpoints registered	33,021	33,317	101	33,210	101
Died	3,948	4,014	102	3,985	101
Reoperation	1,658	1,345	81	1,481	89
debridement	18	0	0	18	100
resection arthroplasty	183	146	80	185	101
revision knee arthroplasty	541	496	92	559	103
secondary patellar resurfacing	303	259	85	303	100
exchange of a knee
prosthesis component	168	131	78	170	101
other revision knee arthroplasty	195	79	41	196	101
arthrodesis	26	13	50	28	108
amputation	22	2	9	22	100
unknown to HDR	202	219	108	–
Septic endpoint	344	298	87	295	86
Aseptic endpoint	1,314	1,047	80	1,186	90
Median length of follow-up,
months (range)	41 (0–103)	42 (0–103)		42 (0–103)

^a The percentage value indicates the proportion of endpoints detected by FAR or HDR, of the number of such first endpoints in the combined endpoint data. Values of > 100% indicate that some of the reoperations were preceded by another type of reoperation detected in the other register (e.g. resection arthroplasty registered in FAR prior to arthrodesis registered in HDR).

Figure 3. Prosthesis survival after primary unicompartmental arthroplasty (UKA), total knee arthroplasty (TKA), and for aseptic and septic revision TKA with any reoperation (including debridement, resection arthroplasty, partial or total revision arthroplasty, arthrodesis, or amputation) due to infection as endpoint. The data on the operations under follow-up were collected from the Finnish Arthroplasty Register (FAR). Endpoint data were collected from FAR and from the Hospital Discharge Register.