Letter to the Editor: Postoperative Infection After Total Hip Replacement With Special Reference to Air Contamination in the Operating Room

To the Editor,

We do understand that letters in response to articles should be written very promptly; however, we would like to comment on a landmark article that you published 50 years ago, in 1972 [1].

The article, “Postoperative Infection After Hip Joint Replacement With Special Reference to Air Contamination in the Operating Room,” by Prof. John Charnley [1], remains of considerable relevance, because it compares the risk of deep infection between standard operating rooms and rooms using so-called “ultra clean air” (UCA) conditions. The paper is particularly useful because none of the patients received antibiotic prophylaxis, it was distinguished by careful microbiological monitoring, and it was a five-year study, with few patients lost to follow-up. The paper also documents and considers a number of confounding variables (such as stopping anticoagulation), but these seemed unlikely to alter the main conclusions.

The paper has a number of hand-drawn charts, which were originally produced for Internal Publication 38 of the Centre for Hip Surgery [2]. These internal publications are still available through the John Charnley Trust, and they are a fascinating read for anyone interested in the history of joint arthroplasty.

We were interested in the data on the association between the use of UCA and late-presenting deep infections in relation to the documentation of infection rates in arthroplasty registries as well as surgical site infection (SSI) registries.

Accordingly, we took the key infection data in Figure 3 from the paper and grouped the patients into those whose procedures were performed in standard operating rooms, with up to 130 air changes per hour, and those treated in UCA enclosures with 300 air changes per hour. We took the infection numbers data by year from Table 7 in the original paper and assumed that the number of patients entering each period decreased by the number of patients with infections occurring in the previous period.

The association between UCA operating rooms and late infection is discernible (Fig. 1). For the entire study span, the increased infection risk associated with standard operating rooms had an odds ratio of 3.3 (95% CI 2.0 to 5.5; p < 0.001) compared with treatment in UCA enclosures. A logistic regression model found differences favoring UCA out to two years after surgery (Table 1). Beyond two years, there were no differences between the groups, with the numbers available.

Fig. 1.

This figure illustrates the number of infections, per 1000 cases, presenting within the various time intervals after surgery, carried out in conventional and ultra clean air operating rooms. A color image accompanies the online version of this article.

Table 1.

Difference between infection rates in conventional and UCA operating rooms within the various time intervals after surgery

Time interval	Infections/1000 conventional	Infections/1000 UCA	Odds ratio (95% CI)	p value
<16 weeks	15	6	2.4 (1.1 to 5)	0.02
16 to 52 weeks	9	3	3.1 (1.1 to 9)	0.04
1 to 2 years	8	0.6	14 (1.8 to 110)	0.01
2 to 3 years	4	1	3.5 (0.7 to 18)	0.13
3 to 4 years	4	2	2.4 (0.6 to 10)	0.24
4 to 5 years	0.8	0	N/A	N/A
5 to 6 years	0.8	0	N/A	N/A

UCA = ultra clean air; N/A = Numbers too small to analyze statistically.

The observation of a reduction in infection rates that persists over time is important in designing SSI monitoring systems, because any infection or arthroplasty registry system that only monitors patients up to one year will miss approximately one-third of the operating room–acquired infections. When monitoring infection rates, there needs to be a balance between failing to record infections related to surgical technique and waiting many years to record low numbers of very late–presenting infections. We suggest that registries should regard infections documented within three years of surgery as treatment complications.