Changes in Invasiveness and Latent Infection Rate Associated with Switching the Approach in Total Hip Replacement.

PURPOSE: Muscle-sparing approaches for total hip replacement (THR) involve learning curves. This study aimed to clarify changes in invasiveness and infection rate with changes in approach.
METHODS: One surgeon changed the approach of THR from Dall's approach (Dall) to anterolateral modified Watson-Jones approach (OCM). Another changed from Dall to a direct anterior approach (DAA). Another 3 surgeons changed from posterolateral approach (PL) to OCM. Subjects were 150 cases, comprising the last 25 cases with conventional approaches and the first 25 cases with new approaches (Dall to OCM: 25 + 25; Dall to DAA: 25 + 25; PL to OCM: 25 + 25 cases). Differences in operative time, bleeding volume, hospital stay, haemoglobin (Hb), white blood cell count, lymphocyte count, creatine kinase (CK) and C-reactive protein (CRP) were investigated.
RESULTS: In the change from Dall to OCM, only hospital stay decreased. In the change from Dall to DAA, hospital stay and CRP decreased, but bleeding volume increased. In the change from PL to OCM, operative time, CRP and CK decreased, but Hb also decreased. Cases with lymphocyte count <1000/μL or lymphocytes comprising <10% of total white blood cells at around day 4 after surgery were defined as latent infection cases. In these cases, operative time was longer, Hb was lower and CK was higher.
CONCLUSION: Introducing muscle-sparing approaches improved many markers of invasiveness, but some items deteriorated. In the early stages of introducing a new approach, choosing cases without obesity and without high muscle volume may reduce the risk of infection.

Introduction

Various surgical approaches have been reported for total hip replacement (THR).[1-4] In recent years, surgical approaches that do not dissect muscles and tendons have been performed at many facilities to minimize the risks of muscle weakness and dislocation after surgery. In fact, muscle-sparing approaches have been reported to involve less pain and lower dislocation rates. In addition, THR with muscle-sparing surgical approaches resulted in faster postoperative recovery and a reduced hospital stay than that with conventional surgical approaches.

However, different surgeries inevitably entail a learning curve. Thus, at the time of switching from a conventional approach to a muscle-sparing approach, both invasiveness and risk of infection may be increased due to extensions in operative time.

THR is one of the most clinically successful orthopaedic surgeries, but the rate of surgical site infection after THR has not been reduced to zero.[9-12] Once periprosthetic joint infection (PJI) becomes established, those bacteria become difficult to suppress, because the bacteria on the implant surface protect themselves by producing glycoprotein biofilms. Detecting and treating surgical site infections at the time of latent infection before PJI thus become decisive. Moreover, cases in which latent infection is likely to occur need to be avoided, particularly when introducing a new approach.

Evaluating the number and ratio of lymphocytes around postoperative day 4 has been reported as a method for detecting latent infection after spinal instrumental operations.[14,15] Even though the infection rate after THR is also very low, the lymphocyte count around postoperative day 4 can be used to detect the latent infection rate and evaluate changes in infection risk.

This study therefore investigated whether invasiveness or latent infection rate change with the approach change, to facilitate effective selection of cases in the initial stage of introducing new approaches to THR.

Materials and Methods

Subjects in this study were 150 patients who underwent THR performed by 5 orthopaedic surgeons in 2 university hospitals between 2015 and 2018. In Facility A, THR was initially performed using Dall’s approach (Dall), but 1 surgeon changed from Dall to an anterolateral modified Watson-Jones approach (OCM). Another surgeon changed from Dall to a direct anterior approach (DAA).[18-20] In Facility B, all 3 surgeons changed from a posterolateral approach (PL) to OCM. This study examined 150 cases, including each of the last 25 cases operated under the conventional approach and each of the first 25 cases operated on using the new approach (Dall to OCM: 25 + 25 cases; Dall to DAA: 25 + 25 cases; PL to OCM: 25 + 25 cases).

For these subjects, we measured operative time, intraoperative bleeding volume, postoperative hospital stays and postoperative haemoglobin (Hb), creatine kinase (CK) and C-reactive protein (CRP) levels to investigate whether the degree of surgical invasiveness and degree of postoperative functional recovery varied depending on the approach. Values before and after changing approach were then compared.

In addition, around postoperative day 4, cases with lymphocyte count <1000/μL and comprising <10% of the total white blood cell (WBC) count were reported to show latent infection.[14,15] After that, it was reported that the lymphocyte count around the fourth day after the operation was useful for determining whether or not the latent infection is occurring.[21-23] Furthermore, it has been actively reported recently that the percentage of lymphocyte to total WBC count or the ratio of lymphocyte to neutrophils indicate latent infection.[24-26] We therefore defined the latent infection as those satisfying either cases with lymphocyte count <1000/μL or cases comprising <10% of the total WBC count around postoperative day 4, and investigated how many such latent infection cases existed and whether any change in the proportion of such cases occurred with the approach change; that is, whether the latent infection rate changed. Specifically, postoperative white blood cell count and postoperative lymphocyte count were measured and compared before and after the change in approach.

Results

By changing from the PL approach to the OCM approach, operative time decreased and CK elevation (postoperative CK-preoperative CK) also decreased, but the amount of Hb reduction (preoperative Hb-postoperative Hb) and postoperative CRP level both increased (Table 1). The change from Dall to OCM only significantly reduced postoperative hospital stay, with no significant differences seen in other items (Table 1). On the other hand, when Dall was changed to DAA, postoperative CRP and postoperative hospital stay both decreased, but intraoperative bleeding volume and amount of Hb reduction increased (Table 1).

Table 1.

Changes in each item associated with switching the surgical approach.

Approach	Item	Conventional approach	Muscle-sparing approach	P-value	Change
PL→OCM	Operative time (min)	142.6	112.1	.0016	↑
	Amount of bleeding (mL)	203	157	.1826
	CRP (1 d) (mg/dL)	5.01	3.84	.1158
	CRP (1 wk) (mg/dL)	2.68	4.35	.0192	↑
	Change in Hb value (g/dL) (1 wk)	−1.76	−2.90	.0021	↑
	Change in CK value (IU/L) (3 d)	+225.9	+130.8	.0323	↓
	Postoperative hospital stays (days)	Not available	Not available	–
	Latent infection rate (%)	12.0	16.0	.6836
Dall→OCM	Operative time (min)	182.5	172.6	.4398
	Amount of bleeding (mL)	540	468	.4976
	CRP (1 d) (mg/dL)	5.07	5.29	.7846
	CRP (1 wk) (mg/dL)	3.54	3.45	.9233
	Change in Hb value (g/dL) (1 wk)	−2.38	−2.22	.6046
	Change in CK value (IU/L) (3 d)	+348.2	+421.7	.4400
	Postoperative hospital stays (d)	45.5	25.5	<.0001	↓
	Latent infection rate (%)	64.0	62.5	.7628
Dall→DAA	Operation time (min)	155.4	149.5	.5014
	Amount of bleeding (mL)	320.5	666.8	.0335	↑
	CRP (1 d) (mg/dL)	5.47	3.75	.0243	↓
	CRP (1 wk) (mg/dL)	3.96	4.11	.8581
	Change in Hb value (g/dL) (1 wk)	−2.38	−1.78	.0312	↓
	Change in CK value (IU/L) (3 d)	+890.7	+186.3	.3912
	Postoperative hospital stays (d)	43.0	28.7	<.0001	↓
	Latent infection rate (%)	32.0	48.0	.3690

Abbreviations: CK, creatine kinase; CRP, C-reactive protein; Hb, haemoglobin.

Change in Hb value: postoperative Hb-preoperative Hb.

Change in CK value: postoperative CK-preoperative CK.

The frequency of latent infection cases (ie, showing lymphocyte count <1000/μL or <10% of the total WBC count around postoperative day 4) did not increase with any approach change (Table 1). Latent infection cases showed longer operative time, higher postoperative CK and lower postoperative Hb than other cases (Table 2).

Table 2.

Comparison of latent infection cases and non-latent infection cases.

Items	Non-latent infection cases	Latent infection cases	P-value	Change
Operative time (min)	142.7	169.5	.0007	↑
Amount of bleeding (mL)	366	507	.0880
CRP (1 d) (mg/dL)	4.47	4.94	.3053
Postoperative CK (IU/L) (1 d)	419	513	.0382	↑
Postoperative Hb (g/dL) (1 d)	10.13	9.58	.0355	↓
Postoperative hospital stays (d)	38.7	33.8	.1284

Abbreviations: CK, creatine kinase; CRP, C-reactive protein; Hb, haemoglobin.

There were no actual cases of infection that met the Musculoskeletal Infection Society criteria[27-29] in the subjects of this study.

Discussion

We investigated changes in latent infection rate due to changes in the THR approach for the first time. As a result, latent infection rate was found to remain unchanged even if the surgical approach changed. However, with the introduction of the muscle-sparing approach, some data were discovered to worsen at first.

In recent years, some muscle-sparing approaches have been actively introduced to institutions for the purpose of achieving both an early return to normal life after hip replacement and higher patient satisfaction. However, such approaches have shown clear technical learning curves as compared with conventional approaches. Thus, when switching from a conventional approach to a muscle-sparing approach, the invasiveness and the risk of infection may increase, for example due to the increased operative time.

Postoperative surgical site infection is the worst complication after total joint replacement. Since PJI is often more difficult to treat than surgical site infection after other orthopaedic surgeries, all orthopaedic surgeons need to know whether the infection rate increases when changing the approach for THR. However, no conclusions can be reached until a considerably large number of patients are investigated, because the postoperative infection rate is very low. If a large number of muscle-sparing approaches are performed, techniques for the new approach will also improve, and any effects of introducing the new approach will be overcome, so whether the risk in infection changes in the early stage of introducing a new technique could not be investigated.

We therefore addressed the above difficulties by investigating the proportion of cases considered to be at risk of infection; that is, the latent infection rate, rather than the actual infection rate. This latent infection rate remained unchanged with the change in approach. However, the amount of bleeding, amount of Hb decrease and postoperative CRP all increased in some cases with the introduction of new approaches. Furthermore, cases with lymphocytes <1000/μL or <10% of WBCs around postoperative day 4, which represent latent infection, had longer operation time, higher postoperative CK and lower postoperative Hb than other cases. In other words, patients with high muscle volume who are likely to experience a long operation time and have high postoperative CK should be avoided at the beginning of introducing the new approach, as should patients with low preoperative Hb who are likely to have low postoperative Hb. Avoiding such patients may reduce the latent infection rate, because bleeding volume is increased and postoperative Hb is decreased in the early stage of introducing a new approach.

One of the limitations of this study is that the concept of ‘latent infection’ has not been established. The latent infection defined by criteria used in this study may simply be a possible sign of immune vulnerability. However, in general, reports on various infectious diseases have shown that neutrophils increase compared to lymphocytes as a biological reaction during bacterial infection.[30-32] Therefore, even if bacterial infection is not manifested, an increase in neutrophil-lymphocyte ratio may represent a latent infection rather than a weakened immune system.

The biggest limitation of this study was that the number of subjects was small. However, we were able to evaluate the state immediately after the introduction of the new approach due to this small number of subjects. In addition, even if the number of subjects was small, clinically useful data could be provided by defining latent infection cases rather than actual infection cases. In the future, a similar study using larger data would establish a clearer method for introducing new approaches. This study offers a first step towards such a study with high clinical relevance.

Conclusion

Many markers of surgical invasiveness were improved with the introduction of muscle-sparing approaches, but some items were aggravated in the early stage of introducing such approaches. Introducing a new approach did not worsen the latent infection rate. However, considering the characteristics of latent infection cases, patients with low muscle mass and high preoperative Hb should be selected in the early stages of introducing a new approach.