Surgical Approach and Microbiologic Trends in Hip Periprosthetic Joint Infection: A Retrospective Comparison of Anterior and Posterior THA

Abstract

Introduction:

Periprosthetic joint infection (PJI) is a serious complication after total hip arthroplasty (THA). Previous work has explored risk factors of infection, but few studies have evaluated whether surgical approach influences the microbiologic profile of THA PJIs.

Methods:

A retrospective review identified patients who developed PJI after primary THA between January 2007 and January 2024. The surgical approach (direct anterior vs. posterior) was determined from surgical reports. Infecting organisms were categorized as gram-positive, gram-negative, polymicrobial, or culture-negative. Clinical and microbiologic variables were compared between approaches. Among culture-positive, microbiologically homogeneous infections, multivariable logistic regression assessed the association between approach and gram-negative infection.

Results:

Eighty-three patients met inclusion criteria: 22 with anterior and 61 with posterior THAs. No significant differences were observed in age, sex, body mass index, Charlson Comorbidity Index, or SIRS score between cohorts (P > 0.05). Organism profiles were broadly similar across groups, with no statistically significant differences in specific pathogens or microbiologic categories, including gram-positive, gram-negative, or culture-negative PJIs. Although not statistically significant, polymicrobial infections were more common in the posterior cohort (13.1 vs. 4.5%, P = 0.433) and gram-negative organisms in the anterior cohort (18.2 vs. 4.9%, P = 0.076). On multivariable analysis, the surgical approach trend toward gram-negative infections remained but was not statistically significant (OR, 3.79; 95% CI, 0.74 to 19.34; P = 0.110).

Conclusion:

The surgical approach was not associated with significant differences in the infecting organism profile after THA PJI. However, the observed trend toward more gram-negative infections with the anterior approach warrants additional investigation.

Periprosthetic joint infection (PJI) remains one of the most clinically and economically burdensome complications after total hip arthroplasty (THA), with profound consequences for patient morbidity, mortality, and the healthcare system.^1-3 While numerous studies have investigated risk factors of PJI, growing evidence suggests that the surgical approach itself may influence not only the risk of infection but also the causative microbiologic profile.^4-6

The direct anterior approach (DAA) has gained widespread popularity because of its muscle-sparing technique, reduced postoperative pain, and potentially faster functional recovery.^7-9 However, its proximity to the inguinal region, a site with dense microbial flora, has raised concerns regarding contamination risk and altered infectious profiles compared with the posterior approach.¹⁰ Emerging evidence indicates a potential association between the DAA and higher rates of gram-negative and polymicrobial PJIs,^6,11 although data remain limited.

Buchalter et al⁶ demonstrated that the surgical approach notably affects the organism profile in THA PJIs, with anterior approaches showing a markedly higher incidence of monomicrobial gram-negative infections. However, their retrospective analysis was limited by the use of a broad “nonanterior” comparator group, rather than a direct comparison between the direct anterior and posterior approaches, and by the absence of multivariable adjustment for potential confounders. Beyond Buchalter's work, the only other study to present detailed organism-level data compared DAA with a lateral approach, not posterior, and found differences in pathogen prevalence, including higher rates of Cutibacterium avidum in DAA PJIs.¹² Furthermore, multiple large database and institutional studies have compared PJI rates across surgical approaches, finding similar or modestly different risks between the direct anterior and posterior approaches, but without providing organism-level data stratified by approach.^13,14 A recent systematic review and meta-analysis of organism profiles in hip PJIs likewise concluded that evidence on approach-specific microbiology remains limited and that additional, approach-stratified studies are needed.¹³ To our knowledge, no previous study has provided a detailed microbiologic comparison of PJIs between the direct anterior and posterior approaches in primary THA.

Given these limitations and the increasing adoption of the anterior approach, additional investigation is warranted. The purpose of this study was to determine whether surgical approach (anterior vs. posterior) is associated with differences in the infecting organism profile among patients with THA PJI. The primary outcome was the distribution of gram-negative versus gram-positive infections. Secondary outcomes included rates of polymicrobial and culture-negative infections. We hypothesized that anterior approaches would demonstrate a higher incidence of gram-negative infections compared with posterior approaches.

Methods

Study Design

After obtaining institutional review board approval, all primary THAs performed at a level 1 academic medical center between January 2007 and January 2024 were screened by manual chart review. Patients who subsequently developed PJI, defined according to Musculoskeletal Infection Society criteria, were included.¹⁵ Exclusion criteria were revision THA at index procedure, hemiarthroplasty, and cases not meeting Musculoskeletal Infection Society criteria.

All diagnoses were verified by manual chart review. Data collected included demographics (age, sex), comorbidities (body mass index [BMI], Charlson Comorbidity Index [CCI], diabetes mellitus, chronic obstructive pulmonary disease, chronic kidney disease, rheumatoid arthritis), and inflammatory response measures (systemic inflammatory response syndrome [SIRS] score at presentation). Surgical characteristics included laterality and surgical approach (anterior vs. posterior). Infectious parameters included organism(s) identified, culture results, and polymicrobial status.

Outcome Measures

The primary outcome was the distribution of infecting organisms in PJIs after THA, stratified by surgical approach. Organisms were classified into broad categories (gram-positive, gram-negative, culture-negative) and specific pathogen subtypes (e.g., methicillin-sensitive Staphylococcus aureus [MSSA], methicillin-resistant S aureus [MRSA], Enterococcus, and Pseudomonas). Polymicrobial infections were further identified and summarized by composition.

Secondary outcomes included the prevalence of polymicrobial infections, the proportion of infections with gram-negative involvement, and the rate of culture-negative PJIs. For polymicrobial infections, the presence of gram-negative organisms and Enterococcus species was further evaluated by surgical approach.

Data Analysis

Descriptive statistics were used to summarize patient demographics, surgical characteristics, and infection profiles. Continuous variables were reported as means with standard deviations while categorical variables were presented as frequencies and percentages. Group comparisons between anterior and posterior surgical approaches were conducted using the chi-square test when all expected cell counts were ≥5 and by the Fisher exact test when any expected count was <5. The Student t test was used for continuous variables.

The primary analysis compared the distribution of infecting organisms between anterior and posterior approach cohorts. Organisms were categorized into predefined groups: gram-positive, gram-negative, polymicrobial, and culture-negative. Secondary analyses evaluated the association of surgical approach with specific organisms (e.g., S aureus, Enterococcus, and Pseudomonas) and polymicrobial status. Individual polymicrobial cases were reviewed and tabulated descriptively.

To assess whether surgical approach independently predicted infection organism type, a multivariable logistic regression model was constructed with gram-negative infection as the binary outcome. The model was adjusted for patient age, sex, BMI, and CCI. Only culture-positive, microbiologically homogeneous infections were included. Four polymicrobial infections composed exclusively of gram-positive organisms were included and classified as gram-positive, as all isolates belonged to the same Gram-stain group.

All statistical tests were two-sided, with significance set at P < 0.05. Analyses were conducted using R version 4.4.1 (R Foundation for Statistical Computing, Vienna, Austria).

Patient Demographics

A total of 2721 primary THAs were performed during the study period. Of these, 83 (3.05%) developed PJIs and were included in the study, comprising 22 with anterior and 61 with posterior surgical approaches. Demographic and comorbidity data for both cohorts are summarized in Table 1.

Table 1.

Patient Demographics and SIRS Score of THA PJI by Approach

Demographics	Anterior (N = 22)	Posterior (N = 61)	P
Age (yr)a	62.55 ± 9.71	62.08 ± 10.65	0.853
Male sex	54.50%	52.50%	1.000
BMI (kg/m2)a	34.31 ± 6.98	33.47 ± 7.71	0.640
DM	13.60%	31.10%	0.160
RA	9.10%	6.60%	0.653
COPD	13.60%	18.00%	0.751
CKD	18.20%	11.50%	0.471
CCI classificationa	2.68 ± 1.67	3.39 ± 2.53	0.145
SIRS scorea	0.45 ± 0.60	0.59 ± 0.84	0.420

^aThe values are given as the mean and standard deviation.

CCI = Charlson Comorbidity Index, COPD = chronic obstructive pulmonary disease, CKD = chronic kidney disease, BMI = body mass index, DM = diabetes mellitus, PJI = periprosthetic joint infection, RA = rheumatoid arthritis, SIRS = systemic inflammatory response syndrome, THA = total hip arthroplasty

On average, patients in the anterior cohort had a slightly higher BMI (34.31 ± 6.98 vs. 33.47 ± 7.71 kg/m², P = 0.640) and a lower CCI (2.68 ± 1.67 vs. 3.39 ± 2.53, P = 0.145), although these differences were not statistically significant. Age (62.55 ± 9.71 vs. 62.08 ± 10.65 years, P = 0.853) and SIRS score at presentation (0.45 ± 0.60 vs. 0.59 ± 0.84, P = 0.420) were also similar between groups. No significant differences were observed in sex (54.5 vs. 52.5% male, P = 1.000), diabetes mellitus (13.6 vs. 31.1%, P = 0.160), rheumatoid arthritis (9.1 vs. 6.6%, P = 0.653), chronic obstructive pulmonary disease (13.6 vs. 18.0%, P = 0.751), or chronic kidney disease (18.2 vs. 11.5%, P = 0.471).

Results

Infecting Organisms by Surgical Approach

Among patients who developed PJIs after THA, the distribution of infecting organisms was similar between anterior and posterior approaches. The most frequently isolated pathogens were S aureus (MSSA and MRSA), coagulase-negative Staphylococcus (CoNS), and Streptococcus species. Complete organism-specific data are given in Table 2.

Table 2.

Specific Infecting Organisms of THA PJI by Approach

Organism	Anterior (N = 22, %)	Posterior (N = 61, %)	P
Coagulase-negative staph	3 (13.6)	12 (19.7)	0.749
Culture-negative PJI	3 (13.6)	7 (11.5)	0.720
Cutibacterium	3 (13.6)	2 (3.3)	0.113
E coli	0 (0.0)	1 (1.6)	1.000
Enterococcus	0 (0.0)	4 (6.6)	0.569
Klebsiella	0 (0.0)	1 (1.6)	1.000
MRSA	1 (4.5%)	6 (9.8)	0.669
MSSA	6 (27.3)	17 (27.9)	1.000
Morganella	1 (4.5)	0 (0.0)	0.265
Other/uncategorized	3 (13.6)	4 (6.6)	0.375
Proteus	1 (4.5)	1 (1.6)	0.462
Pseudomonas	0 (0.0)	1 (1.6)	1.000
Streptococcus	1 (4.5)	5 (8.2)	1.000

MRSA = methicillin-resistant Staphylococcus aureus, MSSA = methicillin-sensitive S aureus, PJI = periprosthetic joint infection, THA = total hip arthroplasty

Grouped by microbiologic classification, gram-positive organisms accounted for most infections in both cohorts, followed by culture-negative and gram-negative PJIs (Figure 1). Normalized percentages were used to visualize organism-level (Figure 2) and category-level (Figure 1) distributions independent of group size. Chi-square analysis demonstrated no significant difference in the distribution of either specific organisms (P = 0.375) or broader categories, including gram-positive, gram-negative, polymicrobial, and culture-negative infections (P = 0.437). Raw counts and statistical comparisons are provided in Table 2 (specific organisms) and Table 3 (broad categories), with no statistically significant differences observed (P > 0.05 for all comparisons).

Figure 1.

Chart demonstrating distribution of broad infecting organism categories in PJI cases after anterior and posterior THA, expressed as percentage of cases by approach. THA = total hip arthroplasty.

Figure 2.

Chart demonstrating distribution of specific infecting organisms in patients who had PJI after anterior and posterior THA, expressed as percentage of cases by approach. PJI = periprosthetic joint infection, THA = total hip arthroplasty.

Table 3.

Broad Infecting Organism Category by THA PJI by Approach

Organism	Anterior (N = 22, %)	Posterior (N = 61, %)	P
Gram-negative	4 (18.2)	3 (4.9)	0.076
Gram-positive	14 (63.6)	43 (70.5)	0.597
Culture-negative	3 (13.6)	7 (11.5)	0.720
Polymicrobial	1 (4.5)	8 (13.1)	0.433

PJI = periprosthetic joint infection, THA = total hip arthroplasty

Specific Infecting Organisms

No statistically significant differences were observed between surgical approaches for any individual organism, including MRSA (4.5 anterior vs. 9.8% posterior; P = 0.669), MSSA (27.3 vs. 27.9%; P = 1.000), CoNS (13.6 vs. 19.7%; P = 0.749), Enterococcus (0.0 vs. 6.6%; P = 0.569), or culture-negative PJIs (13.6 vs. 11.5%; P = 0.720). The “other/uncategorized” category included rare pathogens such as Staphylococcus lugdunensis, Morganella morganii, and Acinetobacter species. No vancomycin-resistant Enterococcus, vancomycin-intermediate or vancomycin-resistant S aureus, acid-fast bacteria, or fungal infections were identified in either cohort.

Broad Infecting Organism Categories

While gram-negative PJIs were more common in the anterior cohort (18.2 vs. 4.9%), this difference was not statistically significant (P = 0.076). No significant differences were found in the rates of gram-positive (63.6 vs. 70.5%; P = 0.597), culture-negative (13.6 vs. 11.5%; P = 0.720), or polymicrobial infections (4.5 vs. 13.1%; P = 0.433) by approach (Table 3, Figure 1).

Polymicrobial Infections

One polymicrobial infection was identified in the anterior cohort, involving two gram-positive organisms: Streptococcus agalactiae and Cutibacterium acnes. Among the eight polymicrobial infections in the posterior cohort, five (62.5%) included at least one gram-negative organism and three (38%) involved Enterococcus. Although none of the polymicrobial infections in the anterior cohort involved gram-negative organisms, the difference in gram-negative involvement between anterior and posterior approaches did not reach statistical significance (0 vs. 62.5%, P = 0.444), likely due to the limited number of polymicrobial cases. Individual organisms are listed in Table 4.

Table 4.

Infecting Organisms of Polymicrobial THA PJI by Approach

Approach	Organism
Posterior (n = 8)
Case 1	Staphylococcus epidermidis, Enterobacter cloacae, Peptostreptococcus
Case 2	MSSA, Proteus mirabilis
Case 3	E coli, Streptococcus agalactiae, Finegoldia magna
Case 4	MSSA, E coli, Pseudomonas aeruginosa, Enterococcus faecalis, Viridans strep, Finegoldia magna
Case 5	2 strains of coagulase-negative Staphylococcus
Case 6	Staphylococcus epidermidis, Enterococcus faecalis
Case 7	Acinetobacter species, Klebsiella species, Enterococcus species, CONS
Case 8	2 strains of coagulase-negative Staphylococcus, Peptostreptococcus
Anterior (n = 1)
Case 9	Streptococcus agalactiae, Cutibacterium acnes

MRSA = methicillin-resistant S aureus, MSSA = methicillin-sensitive S aureus, THA = total hip arthroplasty, PJI = periprosthetic joint infection

Multivariable Analysis

Among 68 patients with microbiologically homogeneous, culture-positive PJIs, multivariable logistic regression showed that surgical approach was not a significant predictor of gram-negative infection (OR, 3.79; 95% CI, 0.74 to 19.34; P = 0.110). No other covariates, including sex (P = 0.419), age (P = 0.896), BMI (P = 0.507), or CCI (P = 0.701), were independently associated with gram-negative infection (Table 5, Figure 3).

Table 5.

Multivariable Logistic Regression Predicting Gram-negative PJI

Predictor	Odds Ratio	95% CI	P
Anterior approach versus posterior	3.79	0.74-19.34	0.110
Male sex versus female	1.04	0.94-1.16	0.419
Age (per year)	1.12	0.20-6.39	0.896
BMI (per unit)	1.04	0.92-1.18	0.507
CCI (per point)	0.9	0.53-1.53	0.701

Multivariable logistic regression model of predictors for gram-negative periprosthetic joint infection (PJI) in patients undergoing total hip arthroplasty. Only microbiologically homogeneous infections were included. BMI = body mass index, CCI = Charlson Comorbidity Index, CI = confidence interval, OR = odds ratio.

Figure 3.

Forest plot displaying ORs and 95% CIs from a multivariable logistic regression model of gram-negative infection among 68 patients who had microbiologically homogeneous, culture-positive PJIs. The model adjusts for surgical approach, sex, age, BMI, and Charlson Comorbidity Index. ORs are shown on a log scale. BMI = body mass index, CI = confidence interval, OR = odds ratio.

Discussion

While previous studies have explored patient-specific and surgery-specific risk factors of PJI, relatively few have examined whether specific surgical approach (anterior vs. posterior) influences infecting organism. In this study, we found no statistically significant differences in organism profiles between anterior and posterior THA PJIs, by either specific pathogen or broad microbiologic classification. However, there was a trend toward an increase in gram-negative organisms in the anterior approach and a higher incidence of polymicrobial infections with the posterior approach.

The most frequently identified organisms across both cohorts were S aureus (MSSA and MRSA), coagulase-negative Staphylococcus, and Streptococcus species, consistent with previous epidemiologic data on hip PJI.^16,17 Gram-positive bacteria accounted for most infections in both groups; however, we observed a notable trend toward higher gram-negative infection rates in anterior THA PJIs (18.2 vs. 4.9%), which approached but did not reach statistical significance (P = 0.076). Although underpowered, this difference may be clinically meaningful and is consistent with previous findings.

Aggarwal et al¹⁸ reported markedly more gram-negative infections in direct anterior approach (DAA) THAs, potentially due to differences in soft-tissue handling, surgical exposure, or proximity to the perineum. This association is biologically plausible, given the anterior incision's location and higher likelihood of contamination with skin and enteric flora. While gram-negative PJIs are often attributed to hematogenous seeding from urinary tract infections, several authors have also reported that gram-negative organisms commonly colonize the gastrointestinal tract, genitourinary tract, and inguinal folds—areas that may be more exposed during anterior surgical approaches—potentially contributing to local surgical site contamination.^1,19-23 These findings suggest that the anterior approach may introduce distinct contamination risks and warrant additional investigation in larger, multicenter cohorts.

Our multivariable analysis, adjusting for age, sex, BMI, and comorbidity burden, did not identify surgical approach as an independent predictor of gram-negative infection. This is consistent with the findings of Ashraf et al, whose systematic review and meta-analysis found no significant difference in organism profiles between DAA and other THA approaches.¹³ However, a key limitation of that study was its binary stratification of DAA versus nonanterior approaches. By grouping posterior, lateral, and anterolateral approaches into a single comparator group, the study introduces substantial heterogeneity, limiting anatomical specificity and interpretability. Recent studies have demonstrated that the lateral approach may carry a higher risk of PJIs compared with the posterior approach.^24,25 Furthermore, the lateral approach exhibits a markedly different microbiologic spectrum and resistance pattern in PJI compared with DAA.¹²

By contrast, our direct comparison of anterior and posterior approaches, those most commonly used in clinical practice, offers more granular, clinically actionable insight.²⁶ This precise stratification enhances internal validity and enables detection of approach-specific microbiologic trends that broader comparisons may obscure.

Institutional variability in perioperative protocols, infection prevention strategies, and laboratory identification techniques also likely contributes to discrepancies between studies. For example, Buchalter et al⁶ implemented enhanced gram-negative antimicrobial prophylaxis and vancomycin intrawound powder (VIP), interventions likely influencing infection profiles. Enhanced gram-negative antimicrobial prophylaxis has been shown to markedly reduce the risk of surgical site infection, with decreases in both gram-negative and gram-positive organisms.²⁷ However, the clinical benefit of VIP seems to depend on concurrent use with systemic cefazolin because data from the study by Ortiz et al²⁸ demonstrate a lack of efficacy for VIP alone without Ancef, highlighting the importance of combination prophylaxis in infection prevention protocols. Such variability is a major confounder in large systematic reviews and meta-analyses. Notably, the review by Ashraf et al did not control for differences in institutional protocols or laboratory methods, further limiting the attribution of microbiologic trends to surgical approach alone.¹³

Among polymicrobial cases, gram-negative organisms and Enterococcus were more commonly involved in posterior THA PJIs, although these differences were not statistically significant. Importantly, only one polymicrobial infection was identified in the anterior cohort, which severely limited the statistical power to detect meaningful differences by approach. This sole anterior case involved two gram-positive organisms: Streptococcus agalactiae and Cutibacterium acnes.

Our study contributes to the growing body of evidence evaluating the microbiologic consequences of surgical technique. Although the anterior approach has been associated with complications such as wound healing issues and lateral femoral cutaneous nerve injury, its role in altering susceptibility to specific pathogens remains uncertain.^29,30 The observed trend toward increased gram-negative infections in anterior THAs merits additional investigation in larger, multicenter cohorts, particularly given implications for empiric antibiotic strategies. Future studies should avoid grouping all nonanterior approaches together and instead stratify by anterior versus posterior to better capture potential microbiologic differences. Care should be taken to standardize or control for institutional variability.

This study is not without limitations. Its retrospective design precludes causal inference, and although our sample size was adequate for detecting broad trends, it may be underpowered for subtle differences in organism frequency. Despite over 2500 THAs performed during the study period, the rarity of PJI limited our sample size. This was particularly evident in the analysis of polymicrobial infections, where only one case occurred in the anterior cohort, severely limiting statistical power to detect approach-related differences in mixed-organism profiles. Although our study included data from multiple surgeons, which may introduce variation in surgical technique and experience, all procedures adhered to standardized institutional protocols, enhancing internal consistency. However, the single-center nature of our data may limit generalizability, particularly given that PJI pathogen profiles can vary markedly by geographic region.⁴

Conclusions

The findings of this study suggest that anterior and posterior surgical approaches do not confer markedly different microbiologic risks in the setting of PJIs after primary THA. While gram-negative infections were more frequently observed in anterior cases, this trend did not reach statistical significance and surgical approach was not an independent predictor of gram-negative infection on multivariable analysis. As of now, these findings suggest that infection prevention and empiric treatment strategies do not support tailoring empiric treatment or prophylaxis based solely on surgical approach. However, additional multicenter studies with larger cohorts are warranted to validate these observations because multiple studies have indicated a trend toward increased gram-negative infections with the anterior approach.