Correction of severe lower limb deformity with digital hexapod external fixator based on CT data

doi:10.21203/rs.3.rs-1708829/v2

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Correction of severe lower limb deformity with digital hexapod external fixator based on CT data

https://doi.org/10.21203/rs.3.rs-1708829/v2

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Purpose: The current study aimed to investigate the therapeutic effect of our self-developed digital six-axis external fixator technique for correction of severe deformities of the lower extremities.

Patients and Methods: From January 2017 to December 2020, our institution used self-developed digital hexapod external fixator technology（QSF）based on CT data to slowly correct 28 severe tibial deformities, and 15 femurs underwent osteotomy and internal fixation. The mean age was 32.6±14.3 years, and the mean follow-up was 23.3±16.1 months. MAD, mFTA, MPTA, LLD, mLDFA at preoperative and final follow-up, and LEFS, KSS and functional score at preoperative and final follow-up were compared.

Results: The QSF adjustment time was 21.4±10.8 days, and the healing time of the tibial osteotomy site was 17.6±7.0 weeks. Preoperative MAD, mFTA, MPTA were 54.1±26.2 mm, 167.7±15.7°, 75.2±12.0°, respectively. At last follow-up, MAD was 8.2±9.9 mm, mFTA was 177.6±3.4°, and MPTA was 87.6±2.4°. Significant improvement was achieved after operation. Preoperative LLD, mLDFA were 13.8±18mm, 83.7±10.8°, at last follow-up were 7.6±7.6mm, 87.8±2.6mm, no significant difference before and after operation. The LEFS, KSS and functional scores improved from 51.6 ± 11.2, 68.5 ± 11.7, 67.8 ± 11.2 to 72.3 ± 6.1, 92.9 ± 3.4, 94.2 ± 6.3, respectively.

Conclusions: The QSF technique can accurately correct severe multiplanar deformities of the tibia in adults. Combined with femoral osteotomy to correct femoral deformity, satisfactory lower extremity alignment can be obtained. The technology has the advantages of simple operation, reliable fixation, less trauma and less complications.

Level of Evidence：Level IV, case series.

Lower extremity deformities may be caused by a variety of traumatic, congenital, developmental, and degenerative diseases. Reconstructing the normal alignment of the lower extremity is a key factor in obtaining satisfactory results after limb reconstruction. Severe lower extremity deformities are often combined with coronal, sagittal and axial planes and shortening deformities, and orthopedic treatment is very challenging. Correction of lower extremity deformity using internal fixation alone has a great impact on the blood supply of the bone, and it is necessary to ensure a successful one-time osteotomy, and postoperative adjustment cannot be performed. The external fixation technique uses the "tension-stress"[1] effect and the "distraction osteogenesis" [2–4] mechanism to gradually correct the deformity, with less trauma and convenient postoperative adjustment [5, 6].

Therefore, external fixation to correct severe deformities of the lower extremity has great advantages over internal fixation, and has achieved great success in recent years. Lower extremity deformities can be corrected with monolateral or circular fixators. Monolateral external fixators are more comfortable and less bulky, but have greater stiffness, and eccentric stress are not beneficial to fracture healing [7], and in many cases they are not suitable for correction of deformities [8]. The Ilizarov frame has a long learning curve, requiring creation of additional hinges when correcting multiplanar deformities [8]. In addition, the disadvantage of the Ilizarov frame is that it is particularly difficult to correct the rotation deformity.

Compared with the Ilizarov frame, computer-aided six-axis frames such as Taylor spatial frame (TSF) can simultaneously correct coronal, sagittal, and axial deformities, which are easy to use and have greater accuracy. It has become the treatment of choice for multiplanar deformities in the last few years [2, 3, 9–12]. However, TSF correction of limb deformity must determine 13 parameters: 3 frame parameters, 4 mounting parameters and 6 deformity parameters. The measurement of every parameter produces errors that will affect the final orthopedic result. and adjustments after TSF application depend on precise anteroposterior and lateral radiographic measurements to produce the most accurate output procedure [13, 14]. These x-rays need to be taken in a plane orthogonal to the reference ring, so X-ray measurements will also have errors due to shooting problems.

We have developed a new six-axis spatial frame technology since 2017, the main structure is similar to TSF, we call it Qiao spatial frame (QSF). The difference between QSF and TSF is that the electronic prescription is calculated based on CT data rather than X-ray data. The aim of our study was to retrospectively analyze the clinical outcomes of severe deformities of the lower extremity treated with this technique.

This study was approved by the ethics reviewing council of Honghui Hospital, Xi’an Jiaotong University, which abides by the Declaration of Helsinki on Ethical principles for medical research involving human subjects (IRB Approval Number 20131201). Written informed consent was obtained from all participants.

From January 2017 to December 2020, a total of 23 cases of lower limb deformity (31 legs) were treated with QSF external fixator in our institution. All patients had at least two deformities (angular and/or rotational, or shortening). Exclusion criteria: (1) lower extremity deformities due to neurological or metabolic diseases. (2) tumor.

One patient was excluded because of multiple osteochondromatosis and one was excluded because of spina bifida. A total of 21 patients with 28 lower extremities were included, including 10 males, 11 females, 16 left lower extremities, and 12 right lower extremities. The average age was 32.6 years (range 13 to 64 years), 13 patients had Blount's disease, 3 had rickets, and 5 had tibial fracture malunion. The mean follow-up time was 23.3 months (Table 1).

Table 1

Patient characteristics
Characteristics
Leg/patients	28/21
Ages(years)	32.6 ± 14.3
Male/female	10/11
Side(right/left)	12/16
Height(cm)	162.0 ± 11.8
Weight(kg)	64.0 ± 12.3
BMI (kg/m²)	24.2 ± 3.1
Follow-up(month)	23.3 ± 16.1
Diagnosis
Blount disease	13
rickets	3
post-traumatic malunions	5

Surgical technique

If there was femoral valgus deformity at the same time, and the mLDFA was greater than 90° or less than 85°, the femoral osteotomy was firstly performed. The apex of the angular deformity (CORA) was selected for the osteotomy site, with 4 osteotomies in the middle of the femoral shaft and 11 in the distal femur. Preoperative use of Solidworks™ software to measure the correction angle and closed gap, Distal femoral varus osteotomy selection of anteromedial incision of the distal femur, and expose the distal medial cortex of the femur. The guide sleeve of the self-designed osteotomy guide was placed close to the medial cortex of the distal femur, and five 2.5mm Kirschner wires was drilled along the sleeve, and the position of the hinge was determined by fluoroscopy to be consistent with the preoperative plan. Maintain continuity of lateral hinge during osteotomy, removed the wedge of bone, slowly close the osteotomy gap until the medial cortex completely fits, and fixed with locking compression plate (TomoFix; Synthes, Solothurn, Switzerland). For valgus osteotomy in the middle of the femur, a lateral incision was selected. The osteotomy guide sleeve is placed close to the lateral cortex of the femur. Four Kirschner wires are inserted, and the osteotomy was performed along the direction of the Kirschner wires. After removing the bone wedge, the osteotomy gap was slowly closed and fixed with locking compression plate (IRENE; Naton, Beijing, China).

Osteotomy of the fibula and tibia began next. A longitudinal incision was made on the lateral side of the middle fibula, and a 1 cm long fibula was cut at the middle and lower 1/3 of the fibula. Transverse osteotomy 1cm below the tibial tubercle, but not completely, to facilitate the installation of external fixators. Choose a C-ring at the proximal end and a full ring at the distal end. According to the deformity of the upper tibia, the configuration of the six-axis external fixator was preset. About 1-1.5cm below the knee joint line, a 2.0 mm diameter Kirschner wire was placed from outside to inside parallel to the tibial plateau, and the position of the proximal ring was adjusted and fixed. Adjust the position of the distal ring to make it perpendicular to the tibial shaft, and fix the distal rings with 4 half pins and proximal rings with 2 to 4 half pins. Completed the osteotomy with the osteotome. Twist the two rings at the distal and proximal ends to ensure complete osteotomy.

The full length of the lower limbs was scanned by CT after operation. Input the CT data into the supporting software to calculate the electronic prescription for six-axis external fixator adjustment. One week after the operation, the adjustment was started to correct slowly according to the prescription, and the speed was 0.7-1mm/d. After discharge, the patients adjusted themselves according to the prescription. After the correction is completed, the full-length AP weight-bearing X-ray of the lower extremity is taken. According to the results, it is determined whether fine-tuning is necessary. After the adjustment of the alignment was satisfactory, the strut was locked. X-rays were reviewed monthly, and the external fixator was removed after the osteotomy site had healed. During the correction process, the patient was instructed to do knee and ankle range of motion exercises to prevent joint stiffness and allow the affected limb to walk with tolerable weight-bearing activities.

Radiographic measures

The lower limb alignment and leg length difference were measured before operation and after osteotomy site healing. The lower limb alignment was evaluated by the mechanical axis deviation (MAD) and the mechanical femorotibial axis (mFTA), tibial alignment by medial proximal tibial angle (MPTA) and femoral alignment by the mechanical lateral distal femoral angle (mLDFA).

Lower limb length was measured using the distance from the top of the femoral head to the center of the ipsilateral ankle joint on the anteroposterior standing whole-leg radiograph. The difference in the length of the lower limbs on both sides was the leg length discrepancy (LLD). All measurements were performed on the picture archiving and communication systems (PACS)(Synapse, Fujifilm Inc., Tokyo Japan). All measurements were performed by 2 observers who did not participate in the operation. After 3 weeks, the measurement was performed one observer again. The intraclass correlation coefficient (ICC) was applied to determine the reliability of the measurement. The ICC values were characterized as follows: poor agreement (< 0.40), fair to good agreement (0.40–0.75), and excellent agreement beyond chance (> 0.75).

Clinical evaluation

Clinical outcome assessment used lower extremity functional scale (LEFS), KSS and functional score before surgery and at the final follow-up.

Statistical analyses were performed using the SPSS version 26 (SPSS Inc., Chicago, IL, USA). All dependent variables were tested for normal distribution using the Kolmogorov-Smirnov test. Paired t test was used to compare MAD, mFTA, MPTA, LLD, mLDFA, LEFS, KSS and functional score before surgery and final follow up. P < 0.05 was considered to be statistically significant.

The Kolmogorov–Smirnov test showed that all data followed the normal distribution pattern. ICC and interclass correlation coefficients for the reproducibility of all parameters were > 80% (Table 2).

Table 2

Intraclass correlation coefficient analysis of measured four parameters before surgery and at last follow-up
	Pre-MAD	Post-MAD	Pre-FTA	Post-FTA	Pre-MPTA	Post-MPTA	Pre- LLD	Post- LLD
Intraobserver	0.95	0.95	0.97	0.98	0.96	0.95	0.97	0.94
Interobserver	0.93	0.91	0.94	0.94	0.93	0.94	90	91

The average adjustment time of the QSF was 21.4 ± 10.8 d, and the time from the end of adjustment to the healing of the osteotomy site was 17.6 ± 7.0 weeks. At last follow-up, MAD was 8.2 ± 9.9 mm, mFTA was 177.6 ± 3.4°, and MPTA was 87.6 ± 2.4°;Preoperative MAD was 54.1 ± 26.2 mm, mFTA was 167.7 ± 15.7°, and MPTA was 75.2 ± 12.0° (Table 3). Compared with preoperative, postoperative improvement was significantly, t were 10.423, -3.592, -5.866, respectively, all P＜0.01. The LLD at the last follow-up was 7.6 ± 7.6mm, and it was 13.8 ± 18mm before operation. There was no significant improvement after operation (t = 1.859, P = 0.074). The mLDFA of the 15 femoral osteotomies at the last follow-up was 87.8 ± 2.6mm, and the preoperative value was 83.7 ± 10.8°. Compared with preoperative, there was no significant improvement (t=-1.630, P = 0.125).

Table 3

Radiological results before surgery and at last follow-up.
Case no.	side	Pre-MAD (mm)	Post-MAD (mm)	Pre-FTA (°)	Post-FTA (°)	Pre-MPTA (°)	Post-MPTA (°)	Pre- LLD (mm)	Post- LLD (mm)	Femoral osteotomy
1	Right	90.38	5.5	152	177	47	86	5.8	3.1	Yes
1	Left	82.14	4.9	156	180	57	86	5.8	3.1	Yes
2	Left	55.2	6	165	177	63	86	30.21	8	Yes
3	Right	75.4	33.6	157	170	66	82	6	15	No
3	Left	48.5	0	164	178	73	87	6	15	No
4	Right	74.2	5	150	178	68	94	4	5	Yes
4	Left	42	1.6	165	181	80	90	4	5	Yes
5	Right	42.6	0	165	179	66	88	2	16	Yes
5	Left	34.4	2.62	168	180	66	89	2	16	Yes
6	Left	39.1	5.7	169	180	73	86	8	6	Yes
7	Right	78.7	17.6	152	174	70	87	0.5	1.2	Yes
7	Left	71.5	5.5	152	179	72	87	0.5	1.2	Yes
8	Right	-30.9	12.2	188	176	86	86	7	8	Yes
9	Right	59.6	16.3	164	177	70	87	27	8	Yes
10	Left	113.3	42.9	138	165	59	87	38	3	No
11	Left	67.1	7.6	159	177	70	88	9	0	No
12	Left	-55.2	12.4	200	176	88	87	42.5	34	Yes
13	Left	36.2	0	171	180	81	90	5	0	No
14	Right	35.4	12.5	170	176	81	89	5	0	No
14	Left	33.1	0	170	179	81	91	5	0	No
15	Right	27.9	8.2	172	178	86	88	1	3	No
15	Left	22.7	11.3	173	178	86	88	1	3	No
16	Right	-118.2	-4	218	181	105	91	85	6.5	Yes
17	Left	47.2	0	167	180	79	89	8.5	12.5	No
18	Left	32	-5	170	181	73	82	12	5	Yes
19	Left	27.6	2.9	171	180	81	88	13.5	6.8	No
20	Right	57.5	7.3	164	178	87	87	22.7	9.4	No
21	Right	-17	0	186	180	92	87	30	20	No
Mean	-	54.1^△	8.2^△	167	177	75	87	13.8	7.6	-
Positive MAD value, mFTA<180°, indicating genu varus, negative MAD value, mFTA>180, indicating genu valgus.

Postoperative LEFS (72.3 ± 6.1) was significantly higher than preoperative (51.6 ± 11.2) (t = 12.285, P = 0.001). The KSS and functional score were significantly improved after the surgery (92.9 ± 3.4, 94.2 ± 6.3, respectively) compared with the preoperative (68.5 ± 11.7, 67.8 ± 11.2 respectively) (t = 13.453, 14.554, respectively, both＜0.001).

All patients' wounds healed within 2 weeks and sutures were removed without nonunion, neurovascular injury, needle tract infection, osteomyelitis and other complications. Only mild discomfort in the calf occurred in the later stage of external frame adjustment. Typical cases were shown in Fig. 1.

Severe deformities of the lower extremities are often accompanied by coronal, sagittal and axial deformities, and it is difficult to completely correct them with internal fixation at one time. Slow correction can completely correct the three-dimensional deformity through tension-stress effects and distraction osteogenesis mechanisms. The single-arm external fixator belongs to the eccentric fixation and cannot effectively resist the rotational stress and vertical deformation capacity. Compared with the acute correction of severe deformities of the lower extremity, gradual correction is more attractive and more effective [15].

The Ilizarov external fixator has been the standard fixator for deformity correction for many years; this fixator is a versatile external fixator that has made a great contribution to deformity correction. Despite its many advantages, correction of complex deformities using the Ilizarov frame requires a long learning curve, and correction of rotational deformities remains quite challenging [16]. Also, when using Ilizarov, it can sometimes be difficult to get the hinges in the correct position. Although the Ilizarov frame could theoretically allow axial correction in every dimension, in practice the treatment of multidimensional deformities may only be partially feasible, and in most cases all deformities can be treated incrementally. This not only increases the operation time, but also easily leads to further deformity [17].

Therefore, the correction of multiplanar and complex deformities with the Ilizarov frame is difficult. Compared to the Ilizarov technique, computer-assisted hexapods have become increasingly popular in deformity correction surgery over the past few years due to their ease of application and accuracy in correcting multiplanar deformities [16]. Ariyawatkul et al. compared the six-axis external frame and the Ilizarov external frame in the treatment of tibial deformity, and concluded that computer-assisted hexapods may reduce the Lengthening Index compared to the conventional Ilizarov method. The hexapods device could be beneficial for faster correction of complex deformity if the patients or family members understand how to manipulate the apparatus [18].

Reitenbach et al evaluated 33 lower extremity deformities treated with TSF and 20 deformities corrected by Ilizarov frame and found that TSF axial deviation and pin infection were significantly less than Ilizarov. The authors conclude that the TSF ring fixator leads to fewer problems, fewer secondary axial translations, and fewer pin infections [12].

TSF is also effective for correction of femoral deformity. Hughes et al [19] used TSF to assist in the correction of 49 femoral deformities, of which 33 were multiplanar, and the authors concluded that a six-axis external frame is a predictable and safe method for femoral deformity correction. In this group, MAD, mFTA, MPTA were significantly improved after operation compared with preoperative, postoperative LLD was less than 2cm, LEFS, KSS and functional score were also significantly improved after operation.

Saw et al [20] treated 22 patients with Blount's disease 32 varus knees using Ilizarov and TSF external frame, preoperative MAD was 95 ± 51.4 mm, FTA 31 ± 15° varus, postoperative MAD 9.0 ± 37.7 mm, FTA 2 ± 14° valgus. Sachs et al [21] reported that the use of Taylor's frame to gradually correct lower extremity deformities, the MAD value from preoperative 51.4mm to postoperative 16.9mm. Li et al [22] reported a more severe deformity with a preoperative MAD value of 90 mm, and after slow correction, the postoperative MAD value reached 10 mm. The preoperative MAD value of this group of patients was 54.1mm, and the postoperative MAD value was 8.2 mm, with an average correction of 47.9 mm.

The QSF developed by our institution based on CT data is composed of 6 mutually angled telescope struts and 2 plane rings connected, similar in structure to TSF. After the QSF is fixed, the computer software is used to process the CT data, and the electronic adjustment prescription is given after calculating the length of the 6 connecting rods that need to be adjusted. However, since its supporting software is based on CT data, the reduction accuracy is higher, and there are no special requirements for the mounting position of the ring and the connection of the struts, so the operation is simpler. Since QSF is based on CT data, there is no need to measure complicated parameters, the realignment result will not be affected by the shooting angle of the X-ray film, so theoretically, the accuracy of correction is higher than that of TSF based on X-ray data. In addition, the advantages of QSF are that it has a short learning curve, can be mastered without professional training, and is firmly fixed. For complex deformity correction, full weight bearing can be achieved postoperatively.

The disadvantage of the study is the small sample size and short follow-up time. Another limitation was that the radiograph measurement did not calculate the magnification rate. According to Sabharwal's study [23], the magnification rate of the standing full-length anteroposterior radiograph length value measurement was 1.05, which meant that our data such as MAD and LLD may be 0.05 times larger than the actual value. Nonetheless, we believe that the QSF technique can effectively treat the severe multiplanar deformity of the lower extremities in adults. However, it is necessary to increase the sample size and conduct long-term follow-up.

QSF technology can accurately correct severe multiplanar deformity of the lower limb in adults. Combined with femoral osteotomy to correct femoral deformity, satisfactory lower extremity alignment can be obtained. The technology has the advantages of simple operation, reliable fixation, less trauma and less complications. It has high clinical application value.

Ethics approval and consent to participate

Consent for publication

The study participants provided written informed consents for their participation and for the use of their data in this study.

Competing Interests

The authors declare no competing interests.

Author contributions

LY, LJ and QF developed the concept, researched the data, and wrote the manuscript. ZB, XZ created the figures and the tables. HJ, QL, WM, JB follow up patients and provide data, FC and CX performed radiograph measurement，All authors have read and approved the final submitted manuscript.

Funding

This study was supported by a grant from the Shaanxi Provincial Key Research and Development Project (CN) (grant no. 2019SF-214) and the Traditional Chinese Medicine Inheritance and Innovation and "Qin Medicine" Development Key Scientific Research Project (grant no. 2021-04-22-006).

Availability of data and materials

The datasets used and/or analyzed in the current study are available from the corresponding authors on reasonable request.

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Correction of severe lower limb deformity with digital hexapod external fixator based on CT data

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