Effect of Femoral Component Flexion Implantation on the Mediolateral Bone‐prosthetic Fit in Total Knee Arthroplasty

Abstract

Objective

Femoral component overhang in total knee arthroplasty (TKA) has been reported in previous studies. The purpose of this study was to evaluate the effect of femoral component flexion implantation on mediolateral bone‐prosthetic fit in TKA.

Methods

Virtual prosthesis implantations were performed on computed tomographic models of 10 Chinese knees with femoral prostheses of the Advance Medial‐Pivot knee system (MicroPort Orthopedics, Arlington, TN, USA), with the femoral component positioned at 0°, 3°, or 6° of flexion in the sagittal plane. For each degree of flexion implantation, the differences between the knee and femoral component models on the lateral and medial sides at trochlea (zone 1), anterior‐distal condyle (zone 2), posterior‐distal condyle (zone 3), and posterior condyle (zone 4) were measured. Positive difference values indicate component overhang, and negative difference values indicate component underhang. The values of component overhang (underhang) in each zone were statistically analyzed across the 3° of flexion implantation.

Results

With a greater degree of flexion implantation, overhang was reduced and even changed to underhang. With 0° of flexion implantation, an overhang exceeding 3 mm existed mainly on the medial side of zone 1 (5.81 mm) and the lateral side of zone 2 (3.39 mm). With 3° of flexion, overhang exceeding 3 mm was observed only on the medial side of zone 1 (3.10 mm), and underhang was observed only on the medial side of zone 4 (−0.32 mm). No overhang exceeding 3 mm was observed for 6° of flexion, while underhang was observed except on the lateral sides of zone 2 (1.32 mm) and zone 4 (1.10 mm) and on the medial side of zone 1 (1.54 mm). A significant difference in overhang values on the lateral and medial sides of zone 1 was observed between 0 and 6° of flexion (P < 0.05).

Conclusion

The present study demonstrated that femoral component flexion implantation by 3° can reduce excessive overhang, although 3.10 mm of overhang remained at the medial side of zone 1. Conversely, 6° of flexion implantation can avoid 3 mm of overhang for any zone, but increases the risk of underhang. Slight flexion implantation may be an effective alternative technique to prevent excessive component overhang, especially in the trochlea and anterior region of the distal condyle, in Chinese patients with standard TKA prostheses.

Introduction

Restoration of knee joint anatomical morphology is a prerequisite for obtaining good postoperative function, and involves prosthetic design, accurate alignment, proper bone cuts, and soft tissue balancing1, 2, 3, 4, 5, 6, 7, 8, 9. Although prosthetic design and surgical techniques have advanced in recent years, bone‐prosthetic mismatch remains a prevalent problem in experimental and practical total knee arthroplasty (TKA), especially in women3, 4, 5, 6, 7, 8. Intra‐articular soft tissue impingement due to component overhang can result in distal femoral osteophytes, extruded bone cement, intra‐articular fibrous bands, and painful irritation of the knee tendons and ligaments10. Component overhang has been demonstrated to be correlated with pain, function, and flexion after TKA7, 8. It was reported that the degree of knee flexion after TKA for patients without mediolateral overhang (127° ± 7°) is better than that of patients with mediolateral overhang (121° ± 11°)7. Among knees with overhang, 39% of outcomes with clinically important pain are attributable to the overhang; a femoral component overhang of ≥3 mm approximately doubles the odds of clinically important knee pain 2 years after TKA8.

Mediolateral overhang can be improved by using a downsized femoral component, but an anterior–posterior dimension that is too small can cause laxity in flexion; balancing of the flexion and extension gaps requires over‐resection of the distal femur to elevate the joint line, leading to an inferior clinical outcome11, 12. The mechanical moment arm of the knee extensor muscle group would theoretically be decreased with a downsized femoral component, which would reduce the extensor strength capacity of the knee after TKA. In addition, component underhang resulting from a downsized femoral component could expose more cancellous bone, which appears to be a source of increased bleeding into the knee in the postoperative period13, 14.

In a study by Bonnin et al. 112 prospectively followed patients with 114 consecutive TKA (64 females and 50 males) were retrospectively assessed; mediolateral overhang was observed in at least one area of the femur in 84% of females and 54% of males7. Mahoney and Kinsey reported that overhang ≥3 mm occurred in at least one zone among 40% (71) of 176 knees in men and 68% (177) of 261 knees in women8. Overhang of the femoral component was highly prevalent, occurring more often in women. The so‐called gender‐specific or morphological‐specific prostheses designed to improve overhang in specific patient groups did not demonstrate very satisfactory results in previous studies4, 5. In fact, one study reported that a gender‐specific knee prosthesis only decreased 6% of overhang in women4, while others reported that the highest femoral component fit rate could be achieved only when both the standard and corresponding gender‐specific or morphological‐specific knee prostheses were available5. To our knowledge, altering sagittal flexion of the femoral component by several degrees during TKA could be a useful downsizing technique15. Slight outliers in the coronal, sagittal, and axial alignment of the femoral or tibial component of TKA have commonly been reported in previous research. Slight sagittal plane flexion angles of 0°–3° for implantation of the femoral component have previously been considered to be acceptable16. We believe this technique might be an effective alternative to decrease mediolateral overhang when only the standard femoral component is available, especially in patients who have relatively narrow femoral condyles. Few studies have reported the effects of component flexion implantation in the sagittal plane on femoral component overhang of either the lateral or medial sides.

In the present study, we performed virtual TKA using computed tomography (CT) knee models of 10 healthy Chinese adults with different degrees (0°, 3°, or 6°) of femoral component flexion implantation in the sagittal plane. Component overhang in four zones of the femoral condyles was measured after the simulated surgery was completed. The purpose of this study was: to evaluate the degree and incidence of component overhang on the lateral and medial sides of the anterior (trochlea), distal, and posterior condyles; to analyze which parts of Chinese femoral condyles are prone to femoral component mediolateral overhang (underhang); and to examine whether component flexion implantation in the sagittal plane would improve overhang on either the lateral or medial sides.

Materials and Methods

Subjects

This study was approved by the institutional review board. Ten healthy southern Chinese adults (four men and six women) recruited for a previous study17 were selected to participate. Participants had an average age of 48.80 years (41–56 years), an average height of 166.30 cm (158–176 cm), and an average body mass index of 24.14 kg/m² (19.10–28.34 kg/m²). The average lower limb mechanical axis was 179.30° (174.67°–183.64°) and the average anteroposterior dimension of the lateral femoral condyles was 61.94 mm (60.91–63.03 mm). Knee joints with deformity, a history of fracture and surgery, or any gonarthrosis were excluded.

The size 2, size 3, and size 4 femoral components of the Advance Medial‐Pivot knee system (MicroPort Orthopedics, Arlington, TN, USA) were used to carry out surgical simulation. The anteroposterior dimension for these components is 57, 62, and 66 mm, respectively. Mediolateral dimension increment is 5 mm.

Data Scanning

The right knees of participants underwent CT scanning (GE ProSpeed CT scanner, GE Healthcare, St Giles, UK) to acquire the datum of the entire femur, with a slice thickness of 0.625 mm and resolution of 512 × 512 pixels. The data of the femoral components (size 2, size 3, and size 4) were acquired by laser scanning (KLS‐171; Kreon Technologies, Limoges, France). Geomagic Studio 10.0 (Geomagic, Morrisville, NC, USA) was used to convert the scanning datum into 3D knee and femoral component models.

Simulated Implantation

For the knee model, the mechanical axis of the femur was defined as the line connecting the center of the femoral head and the center of the knee intercondylar notch. Then, the extreme posterior points of the medial and lateral condyles were aligned in the coronal plane, with the mid‐sagittal plane perpendicular to the coronal plane and passing through the mechanical axis. The distal bone cut was made perpendicular to the mechanical axis in the coronal plane. The femoral component was positioned at 0°, 3°, or 6° of flexion to the plane that was parallel to the anterior cortex of the distal femur in different simulated implantations. The thickness of the distal bone cut was equal to that of the distal condyle of the femoral component (8 mm). The femoral component was set at 3° of external rotation relative to the posterior condylar line and was then shifted as posteriorly as possible without notching the anterior cortex of the distal femur and transversely until the medial‐lateral center of the component reached the mid‐sagittal plane of the knee model.

Selection of the Appropriate Size of Component Model

The femoral component size was selected on the basis of surgical principles. The distance between the anterior femoral cortex and posterior condyle of the knee model (APk) and that between the internal surface of the proximal anterior flange and posterior condyle of the femoral component (APc) were measured. The APk dimensions of the knee models were measured for 0°, 3°, or 6° of flexion implantation (Fig. 1). The APc dimensions for the sizes 2, 3, and 4 components were 52.10, 56.52, and 61.24 mm, respectively. The median value (54.31 mm) between sizes 2 and 3 APc dimensions and the median value (58.88 mm) between sizes 3 and 4 APc dimensions could serve as cutoffs for selecting prosthesis models of the appropriate size. For the match study, if the APk dimension was 54.31–58.88 mm, the size 3 component model was selected. If it exceeded 58.88 mm, the size 4 component model was selected, while if it was less than 54.31 mm, the size 2 component was selected.

Figure 1.

Measurement of APk dimensions of the knee models at different degrees of flexion implantation and measurement of APc dimensions of the femoral component models. Transparent parts represent bones resected after implantation of the femoral component in different simulations. APk, distance between the anterior femoral cortex and posterior condyle of the knee model. APc, distance between the internal surface of the proximal anterior flange and posterior condyle of the femoral component.

Measurements

The transverse distances were measured at four corresponding zones for both the knee and femoral component models after simulated implantation (Fig. 2A). Zones 1–4 correspond to the trochlea, anterior‐distal condyle, posterior‐distal condyle, and posterior condyle, respectively.

Figure 2.

Lateral and axial views of the measurement zones for the knee and femoral component models. (A) Lateral view showing the four measurement zones of the knee and femoral component models. Zones 1 to 4 correspond to the trochlea, anterior‐distal condyle, posterior‐distal condyle, and posterior condyle, respectively. In the knee model, the transparent part indicates bones resected after implantation of the femoral component. (B) Axial view showing measurement of transverse distances on the lateral (a, b, c, and d) and medial (a′, b′, c′, and d′) sides of the four zones in the knee and femoral component models. The component model was made transparent in order for back to be seen.

To analyze component overhang on the lateral and medial sides, the knee and femoral component model was divided into a lateral part and medial part at the mid‐sagittal plane after simulated implantation (Fig. 2B). The transverse distances from the lateral edges of the four zones to the mid‐sagittal plane (a, b, c, and d, respectively) and the transverse distances from the medial edges of the four zones to the mid‐sagittal plane (a′, b′, c′, and d′, respectively) were measured for both the knee and femoral component models, after simulated implantation with different angles of flexion (0°, 3°, and 6°). The gross transverse values of the four zones (a + a′, b + b′, c + c′, and d + d′, respectively) were also calculated for both models. Positive difference values indicated overhang of components beyond the bone edge, and negative difference values indicated underhang.

Statistical Analyses

The differences between the knee and femoral component models on the lateral and medial sides were statistically analyzed across the 3° of flexion implantation using one‐way analysis of variance. Differences in the gross transverse values were similarly analyzed. P‐values of less than 0.05 were considered statistically significant.

Results

Femoral Component Size Selection

With a greater degree of flexion implantation, the required component size decreased. According to the femoral component selection criteria, the component sizes needed were: size 3 (n = 7) and size 4 (n = 3) for 0° of flexion; size 3 (n = 10) for 3° of flexion; and size 2 (n = 4) and size 3 (n = 6) for 6° of flexion.

Component Overhang

With a greater degree of flexion implantation, component overhang was reduced. For the average values of overhang, with 0° of flexion, overhang greater than 3 mm existed mainly on the medial side of zone 1 and the lateral side of zone 2. With 3° of flexion, overhang was observed only on the medial side of zone 1. No overhang was observed for 6° of flexion. Component overhang in zones 3 and 4 was less than 3 mm. A significant difference was observed between 0° and 6° of flexion for zone 1 (Table 1, Fig. 3). For the difference in the average values of gross transverse distances between the knee and femoral component models, the component overhang in zones 1 and 2 was reduced. A significant difference was observed between 0° and 6° of flexion for zone 1 in this case as well (Table 2, Fig. 4). The incidence of overhang (>3 mm) in zones 1° and 2° for the 3° of flexion is shown in Table 3.

Table 1.

Average values of overhang (underhang) on the lateral and medial sides of the four zones for each of the 3° of flexion

Degree	Lateral				Medial
	Zone 1	Zone 2	Zone 3	Zone 4	Zone 1	Zone 2	Zone 3	Zone 4
0°	1.78 ± 2.18*	3.39 ± 2.21	1.67 ± 2.70	2.26 ± 2.74	5.81 ± 3.10*	1.99 ± 2.30	0.91 ± 2.33	0.34 ± 2.17
3°	0.49 ± 1.60	2.21 ± 2.29	1.16 ± 1.98	2.24 ± 2.06	3.10 ± 1.92	0.70 ± 2.11	0.18 ± 1.99	−0.32 ± 1.84
6°	−0.46 ± 1.73*	1.32 ± 2.08	−0.35 ± 2.45	1.10 ± 3.27	1.54 ± 1.93*	−0.43 ± 1.97	−1.04 ± 2.37	−1.36 ± 2.34

Values are expressed as means ± standard deviation; units are expressed in millimeters. Positive difference values indicate component overhang, and negative difference values indicate component underhang.

^* P < 0.05 between 0° and 6° of flexion.

Figure 3.

Variation in the overhang (underhang) values on the lateral and medial sides of the four zones for the 3° of flexion. *P < 0.05 between 0° and 6° of flexion.

Table 2.

Gross overhang (underhang) values of each zone for the 3° of flexion

Degree	Zone 1	Zone 2	Zone 3	Zone 4
0°	7.59 ± 5.02**	5.37 ± 4.36	2.58 ± 4.81	2.61 ± 4.66
3°	3.59 ± 3.17	2.91 ± 4.23	1.34 ± 3.74	1.91 ± 3.43
6°	1.08 ± 3.32*	0.89 ± 3.90	−1.39 ± 4.62	−0.25 ± 5.48

Values are expressed as means ± standard deviation; units are expressed in millimeters. Positive difference values indicate component overhang, and negative difference values indicate component underhang.

^* P < 0.05 between 0° and 6° of flexion.

Figure 4.

Variation in the gross overhang (underhang) values of each zone for the 3° of flexion. *P < 0.05 between 0° and 6° of flexion.

Table 3.

Incidence of overhang (>3 mm) in zones 1 and 2 for the 3° of flexion

Degree	Zone 1		Zone 2
	Lateral	Medial	Lateral	Medial
0°	4/10	7/10	6/10	3/10
3°	0/10	5/10	4/10	1/10
6°	0/10	4/10	1/10	0/10

Component Underhang

With a greater degree of flexion implantation, overhang was reduced and even changed to underhang. No underhang was observed for 0° of flexion. With 3° of flexion, underhang was observed only on the medial side of zone 4. Underhang was prevalent for 6° of flexion (Table 1, Fig. 3). Similarly, with 6° of flexion, underhang was observed from the aspect of gross transverse distances in zones 3 and 4 (Table 2, Fig. 4).

Discussion

Knee joint anatomical morphology is the basis of knee prosthetic design and an important reference for evaluating mediolateral component overhang. Ethnic and gender differences of knee morphology have been proven1, 6, 18. Chin et al. reported that the mediolateral/anteroposterior ratio (aspect ratio) was 1.27 for men and 1.22 for women1. Yue et al. found that knees of Chinese individuals were generally smaller and narrower than those of Caucasian individuals: the aspect ratio was 1.27 ± 0.03 for Chinese men and 1.24 ± 0.04 for Chinese women, whereas the aspect ratio was 1.28 ± 0.07 for Caucasian men and 1.28 ± 0.06 for Caucasian women18. This means that Chinese individuals, especially women, will experience more overhang with standard TKA prostheses.

Differences in population, prosthetic design, and measurement methods have produced different results in studies of component overhang. Yue et al. reported on a Chinese population with femoral component overhang greater than 2 mm in 78% of men and 89% of women when using standard Advance MP Knee (MicroPort Orthopedics) and in 30% of men and 73% of women for Nexgen‐LPS (Zimmer)5. To obtain more detail on overhang, Guy et al. evaluated four zones of the femoral component and found that overhang existed primarily at the anterior flange and anterior‐distal condyle4, which was similar to our results. We found that overhang on the medial side of zone 1 (5.81 mm) and general overhang of zone 1 (7.59 mm) were comparatively severe; there was slight overhang of zone 2 both on the lateral side (3.39 mm) and in the general medial–lateral dimension (5.37 mm). This finding could be explained by the study by Yan et al. wherein the trochlear aspect ratio of the MP femoral component was larger than that of most male and female participants (i.e. for a given anteroposterior dimension, most patients would have mediolateral overhang at the trochlea in TKA)6. Our findings differed from those of Mahoney and Kinsey, who evaluated 10 zones of the lateral and medial sides of the femoral component and found that overhang greater than 3 mm occurred most frequently in the lateral anterior‐distal and distal zones for both men and women8.

A negative correlation of femoral component overhang and pain, function, and flexion in TKA has also been proven. Knee flexion for patients with overhang was worse (121° ± 11°), although no significant difference was observed compared with that in patients without overhang (127° ± 7°)7. Mahoney and Kinsey reported that 39% of cases of postoperative pain were directly related to femoral component overhang8. To improve implant fit and decrease the potential impact of overhang, several gender‐specific or morphological‐specific knee prostheses have been designed, but they are not perfect solutions. Guy et al. reported that overhang for women decreased only 6% when using a gender‐specific knee prosthesis4. Furthermore, Yue et al. showed that the overhang rate was still high when using a morphological‐specific femoral component (26% for men and 39% for women)5. In our study, the method of flexion implantation decreased component overhang effectively: with 3° of flexion, more than 3 mm overhang (3.10 mm) existed only on the medial side of zone 1 and the general overhang was merely 3.59 mm for zone 1, which was smaller in value and less common in zones compared with 0° of flexion. Overhang was less than 3 mm for each zone with 6° of flexion and the occasional underhang could be observed in some zones. One reason for this finding could be the increase of gross transverse distances of the knee models in zones 1 and 2 when simulated TKA were performed with a greater degree of flexion. In addition, for some knee models, with a greater degree of flexion, a smaller component was selected to ensure a suitable match in the simulated TKA. Thus, femoral component flexion implantation could be an alternative to improve overhang and reduce the related negative effects for some patients, when gender‐specific or morphological‐specific prostheses are not available.

An important finding of this study was the component underhang in zones 3 (1.39 mm) and 4 (0.25 mm) at 6° of flexion. Component underhang at the distal and posterior condyles has previously been reported in standard and gender‐specific TKA5, 7, and it could leave cancellous bone exposed and, subsequently, increase bleeding into the knee in the immediate postoperative period, which, in turn, increase the risk of osteolysis from wear debris with time13, 14. Another problem was the restoration of normal posterior condylar offset (PCO), which was considered critical to obtain ideal postoperative range of motion; a reduction in PCO might lead to flexion instability, while excess PCO could cause flexion contracture and limit range of motion (ROM)19, 20, 21. However, a study showed that intentional sagittal flexion of the femoral component by several degrees can be a useful downsizing technique for the femoral component without excessively increasing the flexion gap15. Although the outliers of 0–3° for femoral component flexion in the sagittal plane alignment were acceptable16, the flexion implantation technique should be considered as the last choice to correct component overhang. A previous study showed that increased shape and size offerings of femoral components improved fit during TKA9. Gender‐specific and morphological‐specific prostheses effectively decrease the rate of overhang4, 5. In recent years, the application of customized TKA has been demonstrated to benefit both surgeons and patients22, 23. Therefore, improvements in prosthesis design combined with well‐planned preoperative preparation should be a reasonable solution for bone‐prosthetic mismatch.

The study has some limitations. First, because of the small sample size, the effect of flexion implantation could not be conclusively compared between men and women. Second, only one prosthesis system (the cruciate substitute type knee system) was evaluated and only Chinese individuals were included; therefore, the results may not be generalizable to other prosthetic systems or populations. Third, clinical studies are needed to validate the theoretical results of this study.

Conclusions

The present study demonstrated that femoral component flexion implantation by 3° can reduce excessive overhang, although 3.10 mm of overhang remained at the medial side of zone 1. Conversely, 6° of flexion implantation can avoid 3 mm of overhang for any zone, but increases the risk of underhang. Slight flexion implantation may be an effective alternative technique to prevent excessive component overhang, especially in the trochlea and anterior region of the distal condyle, in Chinese patients with standard TKA prostheses.