Less gap imbalance with restricted kinematic alignment than with mechanically aligned total knee arthroplasty: simulations on 3-D bone models created from CT-scans

Figures & data

Figure 1. LDFA and MPTA comparing preoperative and postoperative distributions.

Table 1. Lower limb alignment of pre-operative anatomy compared with after rKA. Values are mean (SD) [range] degrees.

	Preoperative anatomy	After rKA	p-value	rKA angle modification
				whole cohort	cases modified
HKA angle absolute values	180 (3.6) [168 to 191]	180 (2.2) [177 to 183]	0.6	0.0 (1.8) [–8.3 to 8.8] 1.0 (1.5) [0.0 to 8.8]	–0.1 (2.5) [–8.3 to 8.8] 1.9 (1.7) [0.0 to 8.8)
LDFA absolute values	–2.8 (2.4) [–9.8 to 5.8)	–2.6 (2.1) [–5.0 to 5.0]	< 0.001	0.2 (0.6) [–0.8 to 4.8] 0.2 (0.6) [0.0 to 4.8]	0.4 (0.8) [–0.8 to 4.8] 0.4 (0.8) [0.0 to 4.8]
MPTA absolute values	2.9 (2.6) [–8.9 to 9.9]	2.7 (1.7) [–5.0 to 5.0]	< 0.001	–0.2 (1.6) [–8.3 to 8.8] 0.8 (1.4) [0.0 to 8.8]	–0.4 (2.2) [–8.3 to 8.8) 1.6 (1.7) [0.0 to 8.8]
TEA angle	5.2 (1.8) [0.3 to 9.7)

HKA angle: hip-knee-ankle angle (computed as LDFA + MPTA).

LDFA: lateral distal femoral angle.

MPTA: medial proximal tibial angle.

TEA angle: degrees of external rotation of the transepicondylar axis to the posterior condyles.

rKA angle modification: rKA minus native anatomy.

Figure 2. Distribution of medio-lateral gap imbalance in the extension space for rKA and MA techniques (p < 0.001).

Table 2. Distribution of medial and lateral gap sizes in the extension space for MA and rKA techniques. Values are percentages

Extension space (mm)	Medial extension gap		Lateral extension gap
	MA	rKA	MA	rKA
< 12	17	1.5	17	1.1
12–13	33	5.0	35	5.0
14–15	35	15	32	14
16	15	79	15	80
p-value	< 0.001		< 0.001

The gap size in extension is the sum of the distal femoral bone resection and tibial bone resection.

Note: The aim is for a resection of 16 mm.

Table 3. Medial and lateral gaps modification in the extension space and resulting medio-lateral difference in mm for MA and rKA techniques. Values are mean (SD) [range]

	MA	rKA	p-value
Medial gap	–2.7 (1.9) [–8.9 to 0.0]	–0.4 (1.0) [–6.5 to 0.0]	< 0.001
Lateral gap	–2.7 (1.9) [–9.5 to 0.0]	–0.4 (1.0) [–7.1 to 0.0]	< 0.001
ΔML	0.0 (3.0) [–9.5 to 8.9]	0.0 (1.5) [–7.1 to 6.5]	0.7
absolute values	2.4 (1.9) [0.0 to 9.5]	0.8 (1.3) [0.0 to 7.1]	< 0.001

The gap size modification is the sum of the distal femoral bone resection and tibial bone resection minus 16 mm (resection goal).

ΔML: lateral gap minus medial gap; a negative value in the row represents a greater medial space than lateral space, whereas a positive value represents a greater lateral than medial space.

Figure 3. Distribution of medio-lateral gap imbalance in the flexion space for rKA and MA with PC 3° (p < 0.001) or TEA (p < 0.001) techniques.

Table 4. Medial and lateral gaps modification in the flexion space and resulting medio-lateral difference in mm for MA PC method, MA TEA method, and rKA techniques. Values are mean (SD) [range]

	MA PC method	MA TEA method	rKA	p-value: rKA vs
				MA PC	MA TEA
Medial gap	–1.3 (1.8) [–6.5 to 1.7]	–0.4 (1.9) [–6.8 to 4.3]	–0.4 (1.0) [–6.5 to 0.0]	< 0.001	0.07
Lateral gap	–1.4 (0.6) [–7.3 to –0.9]	–2.4 (1.0) [–7.2 to –0.1]	–0.2 (0.8) [–6.0 to 0.0]	< 0.001	< 0.001
ΔML	–0.1 (2.1) [–8.4 to 5.4]	–2.0 (2.6) [–10 to 5.7]	0.2 (1.4) [–6.1 to 6.5]	< 0.001	< 0.001
absolute values	1.6 (1.3) [0.0 to 8.4]	2.6 (2.0) [0.0 to 10]	0.7 (1.2) [0.0 to 6.5]	< 0.001	< 0.001

ΔML, see Table 3.

Table 5. Flexion–extension gap differences (ΔFE) in mm for the medial and lateral compartments for MA PC method, MA TEA method, and rKA techniques. Values are mean (SD) [range]

	MA PC method	MA TEA method	rKA	p-value: rKA vs
				MA PC	MA TEA
Medial ΔFE	–1.4 (0.6) [0.9 to 6.6]	–2.4 (1.0) [–8.2 to –0.3]	0.0 (0.0) [–0.6 to 0.0]	< 0.001	<0 .001
absolute values	1.4 (0.6) [–6.6 to –0.9]	2.4 (1.0) [–8.2 to –0.3]	0.0 (0.0) [0.0 to 0.6]	< 0.001	< 0.001
Lateral ΔFE	–1.3 (1.8) [–6.5 to 1.7]	–0.3 (1.5) [–6.7 to 4.4]	–0.2 (0.5) [–3.7 to 0.0]	< 0.001	0.005
absolute values	1.8 (1.3) [0.0 to 6.5]	1.6 (1.1) [0.0 to 6.7]	0.2 (0.5) [0.0 to 3.7]	< 0.001	< 0.001

ΔFE: extension gap minus flexion gap; a negative value represents a greater flexion space than extension space, whereas a positive value represents a larger extension than flexion space.

Table 6. Percentage of knees with medial or lateral flexion-extension gap mismatch for MA PC method, MA TEA method, and rKA techniques

	Medial compartment			p-value: rKA vs		Lateral compartment			p-value: rKA vs
	MA PC	MA TEA	rKA	MA PC	MA TEA	MA PC	MA TEA	rKA	MA PC	MA TEA
Ext. gap < 15 mm and flex. gap ≥ 16 mm	5.2	23	0	< 0.001	< 0.001	0	0	4.4	< 0.001	< 0.001
Ext. gap ≥ 16 mm and flex. gap < 15 mm	0	0	0	N/A	N/A	10	9.8	0	< 0.001	< 0.001
Total	5.2	23	0	< 0.001	< 0.001	10	9.8	4.4	< 0.001	< 0.001

Table 7. Percentage of knees where the medio-lateral gap mismatch is present in both the extension and flexion spaces for MA PC method, MA TEA method and rKA techniques

				p-value: rKA vs
Gap mismatch	MA PC	MA TEA	rKA	MA PC	MA TEA
≤ 3 mm	63	49	92	< 0.001	< 0.001
≤ 5 mm	89	81	99	< 0.001	< 0.001
> 5 mm	1.9	3.8	1.1	0.1	< 0.001

Figure 4. Lower limb long radiographs showing a case with an LDFA of 11° and MPTA of 6°. Reproducing her lower limb alignment with KA technique (unrestricted) would leave her lower limb HKA in 5° of valgus. With rKA, correcting the femur to 5° and the tibia to 2° of varus would results in an HKA of 3° valgus.