No influence of obesity on mid-term clinical, functional, and radiological results after computer-navigated total knee arthroplasty using a gap balancing technique

Raj Kanna; Ananth Brasanna; Gautam M Shetty; Chandramohan Ravichandran

doi:10.1016/j.jcot.2021.01.001

. 2021 Jan 8;16:136–142. doi: 10.1016/j.jcot.2021.01.001

No influence of obesity on mid-term clinical, functional, and radiological results after computer-navigated total knee arthroplasty using a gap balancing technique

Raj Kanna ^a,^∗, Ananth Brasanna ^a, Gautam M Shetty ^b, Chandramohan Ravichandran ^a

PMCID: PMC7920157 PMID: 33717948

Abstract

Objective

In obese patients, thick subcutaneous tissue can introduce errors during registration and leg weight can influence gap balancing in navigated TKA. Present study is done to determine if computer navigated TKA using a gap balancing technique can achieve consistent accuracy for limb and component alignment, and similar clinical and functional results in obese patients like in non-obese patients.

Methods

We prospectively compared the radiological, clinical, and functional results of 78 knees in 57 non-obese patients and 79 knees in 58 obese patients who underwent computer-assisted TKA. Non-obese individuals were defined as those having BMI of <30 kg/m² and obese individuals as BMI ≥30 kg/m². The degree of knee deformity was calculated by Hip – Knee – Ankle (HKA) angle and clinical and functional assessment was done using the Knee Society Score – clinical knee score and Knee Society Score - function score, respectively. All these were documented before and at 6 months, 2 year, and 5 years after TKA.

Results

The outlier rate of postoperative limb alignment (HKA angle) was 8.9% in the obese group which was not significantly different (p =1.00) from that of the non-obese group (7.7%). Mean clinical knee scores were not significantly different between the non-obese and obese groups preoperatively (58.8 vs 57.4, p = 0.14) and at 6 months (92.7 vs 91, p = 0.06), 2 years (91.4 vs 90, p = 0.07), and 5 years (92.4 vs 91.3, p = 0.1) post-surgery. Similarly, mean functional scores were not significantly different between the non-obese and obese groups preoperatively (50.9 vs 49.9, p = 0.31) and at 6 months (92.7 vs 90.9, p = 0.06), 2 years (91.3 vs 92, p = 0.44), and 5 years (90.6 vs 91.1, p = 0.51) post-surgery.

Conclusion

Obesity has no influence on mid-term clinical, functional, and radiological results after computer navigated TKA, done by gap balancing technique.

Level of evidence

Therapeutic level II.

Keywords: Computer-assisted knee replacement, Limb alignment in obesity, Knee navigation in obesity, Functional results in obesity, Gap balancing technique

Abbreviations

BMI: Body mass index
GB: Gap Balancing
TKA: Total Knee Arthroplasty
HKA: Hip knee ankle
KSS: Knee Society Score
KSKS: KSS - clinical knee score
KSFS: KSS - function score
TT: Tourniquet time
OKS: Oxford Knee Score

1. Introduction

Literature is divided over the influence of obesity on outcome in TKAs. While some authors have reported greater risk for limb mal-alignment in TKAs when performed in the obese,¹^,² others have shown no increase in risk for mal-alignment in the obese patients undergoing TKA.³^,⁴ Similarly, some studies have showed comparable clinical and functional results between obese and non-obese patients⁴^,⁵^,⁶ and others have concluded obesity to have a negative influence on the clinical and functional outcome after TKA.⁷^,⁸^,⁹

Over the years, studies have established the superiority of computer-assisted TKA in achieving the desired component and limb alignment in comparison to that achieved by conventional TKA.¹⁰^,¹¹^,¹²^,¹³^,¹⁴^,¹⁵^,¹⁶^,¹⁷ But when it comes to functional outcome after navigated TKA opinions are divided. While various authors¹⁸^,¹⁹^,²⁰^,²¹^,²² have established the functional benefits of navigated TKA compared to conventional TKA, others²³^,²⁴^,²⁵^,²⁶ have refuted this. Nevertheless, while performing navigated TKA in obese patients, errors can occur during registration of femoral head centre, because of excess soft tissue in the lower abdomen, hip, and the thigh (Fig. 1). Further, it may be difficult to register the ankle centre as palpating the malleoli may not be easy. In addition to this, during GB technique, spring tensioner device is used to open the gaps in extension and flexion. In obese patients there is a chance for introducing errors because of the weight of the leg, which can influence the gap values in extension and flexion (Fig. 2). This can affect the size and position of the femur component which can influence the outcome.

Fig. 1 — Obesity with excess soft tissue in the lower abdomen, hip, and the thigh can be a potential cause for error during femoral head centre registration in navigated TKA.

Fig. 2 — Registration of extension and flexion gaps using spring-loaded tensioner device (arrow) in navigated TKA.

Therefore, with the rising number of obese patients, and paucity of literature on the influence of obesity on radiological²^,²⁷^,²⁸ and functional²^,²⁷ outcome after navigated TKA, the present study was done to a) determine if computer navigation provides consistent accuracy for limb and component alignment in obese sub-group of patients like in non-obese patients and b) to know if navigation achieves similar clinical and functional results in obese and non-obese patients when GB technique is used, at the end of 5 years after TKA.

2. Material and methods

2.1. Study design, settings, and participants

We prospectively studied the clinical, functional, and radiographic results of 187 consecutive computer-assisted TKAs performed between 2012 January and 2015 January, in 136 patients. The inclusion criteria was patients who underwent primary, cruciate-substituting, computer-assisted TKAs for primary osteoarthritis of the knee. We excluded four knees with rheumatoid arthritis, one limb with extra articular deformity, two knees with old fractures, three knees which were operated before, four knees where constrained implants were used and five revision knee replacement surgeries from the current study. Finally, patients who were lost to follow up for various reasons were excluded from the present study (Fig. 3). At the time of final follow up, complete data of 78 computer-assisted TKAs performed in 57 non-obese patients and 79 computer-assisted TKAs performed in 58 obese patients were taken for analysis. Non-obese individuals were defined as those having a body mass index (BMI) < 30 kg/m² and obese individuals were defined as those having a BMI ≥ 30 kg/m². Demographic parameters in the non-obese and obese groups are summarised in Table 1.

Table 1.

Comparison of demographic parameters between non-obese and obese groups.

	Non-Obese Group	Obese Group	P value
Number of limbs (Patients)	78 (57)	79(58)
Males	17 (29.8%)	23 (39.7%)	0.32
Females	40 (70.2%)	35 (60.3%)
Age (Years)	64.5 ± 8.8 (46–82)	62.2 ± 7.3 (51–79)	0.12
BMI (kg/m²)	25.6 ± 2.3 (18–29.9)	34 ± 4.2 (30–44.1)	<0.0001
Tourniquet time (minutes)	80.9 ± 12.7 (58–101)	86 ± 12.6 (61–118)	0.01

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BMI – body mass index; All data presented as mean ± standard deviation (range) or number(percentage); p < 0.05 is considered statistically significant (bold).

2.2. Variables

Both height and weight were recorded by a ward nurse preoperatively on admission of the patient to the hospital. The height was measured using a wall mounted measuring scale and the weight was measured using a weighing scale. Height (m) and weight (kg) were then used to derive BMI (kg/m²) in each patient.

Standing full length (hip to ankle) weight-bearing radiographs were obtained in all patients both before and within 3 weeks after surgery. The degree of knee deformity or hip–knee–ankle (HKA) angle was determined on the standing full-length radiographs as the angle between the mechanical axis of the femur (centre of the femoral head to the centre of the knee joint) and the mechanical axis of the tibia (centre of the knee joint to the centre of the ankle) (Fig. 4). Weight-bearing anteroposterior knee radiographs and knee lateral radiographs were obtained in all patients before and at 6 months, two years, and five years after TKA.

Fig. 4 — Leg alignment before (a) and after (b) computer-assisted TKA in an obese patient (BMI 39.8) and full leg standing scanogram used for measuring limb alignment (HKA angle) before (c) and after (d) computer-assisted TKA.

Postoperatively, coronal alignment of femoral and tibial components was measured using their respective mechanical axes on full-length radiographs (Fig. 5). Limbs with postoperative HKA angle outside the acceptable ±3° range from a neutral alignment of 180° were considered outliers for limb alignment. Similarly, components outside the acceptable ±3° range from a neutral alignment of 90° in the coronal plane were considered outliers for component alignment. Radiographs taken preoperatively and at 6 months, two years, and five years after TKA were scrutinized for radiolucent lines, and signs of loosening.

Fig. 5 — Measuring coronal alignment of femoral and tibial components using their respective mechanical axes on full-length radiograph.

Active knee flexion was measured using goniometer with the patient in supine position before and at 6 months, two years, and five years after surgery. Clinical and functional assessment was done by the Knee Society Score (KSS) (Insall, 1989), which is divided into two sections: a clinical knee score (KSKS) and a function score (KSFS). The scores were documented before and at 6 months, two years, and five years after TKA.

All TKAs were performed by a single surgeon using the computer-assisted technique. All procedures in the non-obese and obese groups were performed with the tourniquet which was inflated just before skin incision and deflated soon after the cement had hardened. An anterior longitudinal incision and a medial parapatellar arthrotomy were used in all cases. All patients underwent TKA using a cemented, fixed bearing, posterior cruciate substituting design and all patients had resurfacing of the patella. The P.F.C. Sigma implant (DePuy Orthopaedics, Warsaw, Indiana) was used in all patients. Tibial stem extenders were used in 7 knees. In 4 knees it was used in addition to bone grafting and in the other 3 knees, stems were used to withstand the weight of morbidly obese patients. The surgical aim in all patients of both groups was to align both femoral and tibial components perpendicular to the respective mechanical axes, to achieve femoral rotational alignment parallel to tibia cut, and to attain a neutral lower limb mechanical axis.

We used the Ci navigation system with its software (Version 2.1, Brainlab, Munich, Germany). Using 2 infra-red arrays affixed to the tibia and femur each, the centre of femoral head was computed by pivoting the femur within the free range of motion of the hip joint. To minimise errors during registration of the femoral head centre, care was taken to ensure that the pelvis was properly stabilised at the anterior superior iliac spine by an assistant and the hip joint moved through a smooth, gradual circulatory movement mimicking the base of a cone. Following standard registration process, the mechanical axis of the limb was determined by the navigation software. Conventional cutting blocks were navigated into position to perform the appropriate bone cuts. The degree of soft tissue release was governed by the amount of soft tissue tightness and medial and lateral gap imbalance as quantified by the computer. We used GB technique to determine the femoral component rotation. Once the tibial cut had been made with navigation, the flexion gap and extension gap were measured after distracting the gaps with the spring-loaded tensioner device (Fig. 2). The femoral component size, anteroposterior position, flexion, and rotation were determined precisely using navigation, to obtain a 1–2 mm looser flexion gap.

2.3. Statistical analysis

Sample size calculation using the primary variable post-treatment KSS knee score was performed. Setting the type-I error at 0.05, power of the study at 80%, the minimum number of 63 knees was calculated as sample size in each group. Radiographic parameters of the non-obese group were compared to those in the obese group using the paired t-test. The percentage of outliers for limb and component alignment was compared between the groups using the Fisher’s exact test. Tourniquet time (TT), Knee flexion, KSKS and KSFS was compared between the groups using the paired t-test. A p value of <0.05 was taken to be statistically significant.

3. Results

Radiographic parameters in the non-obese and obese groups are summarised in Table 2. Mean pre (166.4 ± 8.8 vs 167 ± 8.1) and postoperative (179.3 ± 1.9 vs 179 ± 1.5), limb alignment i.e HKA angle was not significantly different between the non-obese and the obese groups. Similarly, mean coronal alignment of the femoral and tibial components were not significantly different between the non-obese and the obese groups (Table 2). The percentages of outlier for postoperative limb alignment, coronal alignment of the femoral and tibial components between the non-obese and obese groups were not significantly different (Table 2). None of the knees in the non-obese and obese groups showed progressive radiolucent lines or loosening in the postoperative radiographs until 5 years post-surgery.

Table 2.

Comparison of radiographic parameters between non-obese and obese groups.

Parameters	Non-Obese Group	Obese Group	P value
Pre Op HKA Angle (Degrees)	166.4 ± 8.8 (150–189)	167 ± 8.1 (150–190)	0.642
Post Op HKA Angle (Degrees)	179.3 ± 1.9 (175–185)	179 ± 1.5 (175–184)	0.250
Femoral Component (Degrees)	90.4 ± 1.6 (86–94)	89.9 ± 1.7 (85–94)	0.071
Tibial Component (Degrees)	90.6 ± 1.4 (86–94)	90.2 ± 1.7 (86–94)	0.13
Limb alignment outlier	7.7%	8.9%	1
Femur component outlier	9%	11.4%	0.8
Tibial component outlier	6.4%	6.3%	1

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HKA – hip-knee-ankle; All data presented as mean ± standard deviation (range).

Although the mean knee flexion was marginally less in the obese group compared to the non-obese group both before and at 6 months, two years, and five years (130.3 ± 15.9 vs 134 ± 9.4) after surgery the difference in the mean knee flexion between the groups were not statistically significant (Fig. 6). Both non-obese and obese groups showed a significant increase in the KSKS and KSFS in the postoperative stages compared to the respective pre-operative scores. However, the KSKS (Fig. 7) and KSFS (Fig. 8) between the non-obese and obese groups were not significantly different, both before and at 6 months, two years, and five years (KSKS - 92.4 ± 4.2 vs 91.3 ± 4.6; KSFS – 90.6 ± 5.3 vs 91.1 ± 5.5) after surgery.

Fig. 6 — Mean active knee flexion before surgery and at six months, two years and five years post-surgery.

Fig. 7 — Mean knee society knee score (KSKS) before surgery and at six months, two years and five years post-surgery.

Fig. 8 — Mean knee society function score (KSFS) before surgery and at six months, two years and five years post-surgery.

Six navigated TKAs were done in four patients who were morbidly obese (BMI ≥ 40 kg/m²). Post-operative HKA angle outside the acceptable ±3° and coronal mal-alignment of the femur component >3° was seen in one knee. Coronal alignment of the tibial components were within the acceptable range (≤3°) in all the knees. At five years post-surgery the mean knee flexion, KSKS, and KSFS of this subset of patients were 125.5° ± 6.1°, 89.7 ± 4.6, and 89.2 ± 7.4, respectively.

One patient (in non-obese group) who underwent unilateral TKA developed deep infection three weeks after surgery which was treated by debridement and exchange of polyethylene insert and had full recovery. None of the knees in the obese patients developed deep infection. None of the knees had navigation related complications such as, pin tract infection or pin site fracture.

4. Discussion

When it comes to functional outcome, the superiority of navigated TKA over conventional TKA is still debated. However, several studies have shown that, functional results after navigated TKA, done by GB technique were significantly better compared to that of conventional TKA.¹⁸^,¹⁹^,²⁰^,²² Similarly, a recent meta-analysis²⁹ although, showed no significant differences in outcomes between navigated TKA that controlled only limb and component alignment versus conventional TKA in fact, showed significant difference in outcome between navigated TKA that also controlled soft tissue balancing versus conventional TKA.

In navigated TKA obese patients may be prone to errors, due to difficulty in registering the femoral head centre because of soft tissue impingement (Fig. 1). Also, there may be difficulty in registering the ankle centre because, palpating the malleoli may not be easy. Furthermore, excessive fat makes exposure difficult in the obese patients and excessive pressure due to the subluxated or everted patella or due to the thick medial flap may cause difficulty in palpating and registering the femoral epicondyles. Lastly, the weight of the leg might affect the soft tissue tension and therefore the gaps in extension and flexion which can influence the outcome (Fig. 2).

Our study shows that computer-assisted TKA resulted in equally excellent limb and component alignment in obese as in non-obese patients. The outlier rate for postoperative limb alignment (HKA angle) was 8.9% in the obese group which was not significantly different from the 7.7% outlier rate in the non-obese group. Shetty et al.²⁸ in their study concluded that the mean limb and component alignments were not significantly different in the non-obese and obese groups after navigated TKA. The percentage of outliers for the limb alignment in their study (6.2% for non-obese and 7.5% for obese patients) was similar to that in the current study. Similarly, Kamat et al.² and Puah et al.²⁷ in their studies, showed that the percentage of outliers for the limb alignment were not significantly different between the non-obese and obese groups after navigated TKA.

The present study shows that the mean KSKS and KSFS scores were not significantly different between the non-obese and obese groups until five years after navigated TKA. Similarly, Kamat et al.² compared the outcome scores between non-obese and obese patients undergoing computer-assisted TKA and showed that the Oxford Knee Score (OKS) was not significantly different at five years and KSKS and KSFS scores were the same between the groups at three years post TKA. In the current study we used GB technique to determine the femoral component rotation. Similarly, Puah et al.²⁷ in their study used navigated GB technique and showed that the mean OKS was similar in both non-obese and obese patients at two years after surgery. When it comes to functional outcome, ours is the largest series to date, with a total of 157 navigated TKAs reported in the study. Whereas, in the previous studies by Kamat et al.² and Puah et al.,²⁷ the total navigated TKAs reported were relatively less i.e., 133 and 101, respectively and in the later series only 23 TKAs were included in the group with BMI ≥30 kg/m².

The mean TT in our study was significantly less (p = 0.01) in the non-obese group compared to that of the obese group, i.e., 80.9 ± 12.7 vs 86 ± 12.6. Similarly, Kamat et al.² showed that mean TT was 88.7 ± 20 in the non-obese group and 94.2 ± 17.5 in the obese group. This difference was nearing significance (p = 0.09) even though the number of navigated TKAs in their obese and non - obese groups were relatively small (64 and 69 respectively) compared to that of ours. The mean TT of the total 157 navigated TKAs done in our study was 83.5 ± 12.9 min, which was comparable to the mean TT of the navigated TKAs (90 and 85 ± 22 min) and slightly higher compared to that of the conventional TKAs (75 and 65 ± 17 min) reported in the past studies by Conteduca et al.³⁰ and Hsu et al.,³¹ respectively.

The present study has certain limitations. KSS has higher ceiling effect and recent studies have shown better outcome evaluation tools with low ceiling effect which may be ideal measuring tools for evaluating patient outcome after TKA.³²^,³³ Secondly, although equally good functional outcome was achieved in obese patients in the current study following navigated GB technique, a prospective randomised study comparing the results of navigated GB and navigated measured resection technique in obese subset of patients will actually demonstrate the superiority of one over the other in this subset of patients. Lastly, comparison of the outcome of the morbidly obese subset of patients was not possible because of the smaller size of this group.

The fraction of TKA patients falling in the obese category keep increasing day by day and the results of conventional TKA in obese patients are still debated by some. The present study shows that despite intraoperative difficulties, similar radiological, clinical, and functional outcomes can be achieved in obese patients, like in non-obese patients up to 5 years after navigated TKA with gap balancing technique. Further long-term studies comparing the radiological and functional results of navigated and conventional TKA, in obese subset of patients should confirm the real benefits of navigation compared to conventional TKA in this subset of patients.

Funding

No funding received.

Declaration of competing interest

None.

Acknowledgements

Not applicable.

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PERMALINK

No influence of obesity on mid-term clinical, functional, and radiological results after computer-navigated total knee arthroplasty using a gap balancing technique

Raj Kanna

Ananth Brasanna

Gautam M Shetty

Chandramohan Ravichandran