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Restoration of proper joint line position after primary total knee arthroplasty (TKA) is important for improved knee function and kinematics. We reviewed the magnitude of joint line alteration and the resulting effect on post-operative knee outcomes scores at one year follow-up.
Materials and methods
120 patients who underwent TKA for primary osteoarthritis knee were included. Assessment of joint line position before and after surgery was performed with the help of bony landmarks (excluding osteophytes): Medial Epicondyle Joint Line distance (MEJL), Lateral Epicondyle Joint Line distance (LEJL) and Fibula Head Joint Line distance (FHJL) were calculated. Post-operative knee function was assessed using ‘The Western Ontario and McMaster Universities Arthritis Index’ (WOMAC) and ‘Knee Society Score’ (KSS) at one year follow-up.
Results
Joint line elevation was observed in 104/120 (86.7%) patients and 16/120 (13.3%) patients had no joint line elevation. The mean joint line elevation was 3.00 (±2.13) mm. The patients were sub-classified into two study groups: Group A- Joint line elevation <5 mm and Group B- Joint line elevation ≥ 5 mm. The mean post-op one-year KSS score was significantly higher in patients in Group A compared to Group B (52.82 ± 7.564 vs. 40.73 ± 7.146; p < 0.001). The mean post-op one-year WOMAC score was lower in patients in Group A compared to Group B (65.51 ± 14.762 vs.75.64 ± 8.203) and the difference was statistically significant (p = 0.002).
Conclusion
Elevation of joint line ≥5 mm from the pre-operative value has a negative impact on post-operative functional outcome in primary TKA.
Total knee arthroplasty (TKA) is a superior operative management modality in patients having osteoarthritis of knee. The rising trend in the prevalence of TKA is due to a more demanding active elderly population leading to a dramatic rise in the numbers of primary knee arthroplasties performed annually.
The dramatic increase in total kneereplacement utilization rates in the United States cannot be fully explained by growthin population size and the obesity epidemic.
Matched resection of bone in primary TKA may be useful to achieve this goal unless associated with bone loss due to prior surgery or implant failure. The elevation of mean joint line after primary TKA varies between 1.1 mm and 5.6 mm and it is reported as much as 8 mm in revision cases.
Elevation of joint line by 5 mm or more have a detrimental effect due to complications such as mid-flexion instability, patellar impingement, decreased range of motion (ROM), quadriceps weakness and increased patellofemoral contact forces.
Joint line positioncorrelates with function after primary total knee replacement: a randomised controlledtrial comparing conventional and computer-assisted surgery.
The present study aimed to analyse the magnitude of joint line elevation after primary TKA, examine the effect of joint line alteration on outcome measures and to propose a cut-off limit for post-operative joint line elevation following primary TKA.
2. Materials & methods
The study was carried out in a single tertiary care hospital after taking requisite approval from the Department Review Board (DRB) and the Institutional Ethics Committee (IEC). All patients who underwent TKA for primary osteoarthritis (OA) of knee with minimum follow-up of one year were included in the study. Complex primary TKA (TKA post high tibial osteotomy, malunited fracture, congenital deformities), revision TKA, cases with rheumatoid arthritis and those who denied consent for participation were excluded from the study. Those patients who had pre-operative flexion deformity of >20⁰ were excluded from the study as it may affect the radiographic joint line assessment on standing X-ray. Using the above inclusion and exclusion criteria, a total of 120 patients (46 males and 74 females) were included in the study (operated and assessed between January 2017 and December 2019). A well-informed, written consent was obtained from all patients enrolled for the study. Assessment of joint line position on pre-operative X-rays were performed by two independent observers (one orthopaedic senior registrar and one orthopaedic resident) with the help of anatomical bony landmarks (excluding osteophytes) to determine: a.) Medial Epicondyle Joint Line distance (MEJL), b.) Lateral Epicondyle Joint Line distance (LEJL) and c.) Fibula Head Joint Line distance (FHJL) on a true size standing anteroposterior (AP) radiograph of the knee with patella centred over the femoral condyles (Fig. 1).
Fig. 1Pre-operative assessment of joint line position with respect to anatomical bony landmarks (medial epicondyle, lateral epicondyle and fibula head) on a standing AP radiograph. Note that the osteophytes were excluded while performing these measurements.
The joint line was drawn by a tangent which connects the distal most points of both the femoral condyles. Medial epicondyle joint line distance is the perpendicular distance from the superior edge of medial epicondyle sulcus and the joint line. Lateral epicondyle joint line distance is the perpendicular joint line distance from the most prominent edge on the lateral epicondyle to the joint line. Fibula Head Joint Line distance is the perpendicular distance from the highest point of fibular head to the joint line. All observations were carried out on two separate occasions by both the observers separated by a period of one week and an average value was calculated which was rounded up to the next higher integer. All radiographs were taken by a single senior radiology technician using a constant marker (metal coin of known diameter) to maintain uniformity. On an average, joint line position in a normal knee has been observed to be around 35 mm from the medial epicondyle, 25 mm from the lateral epicondyle and 10 mm proximal to the fibula head on AP radiographs.
All surgeries were performed by the senior-most orthopaedic surgeon using a medial parapatellar approach. Fixed/Mobile bearing cruciate-substituting prosthesis (DEPUY PFC-Sigma Knee) was used in all patients. A combination of measured resection & gap balancing techniques were used to obtain symmetrical rectangular flexion and extension gaps. Anterior referencing anteroposterior cutting block was used for anterior and posterior femoral condylar resection. The patellar resurfacing was done in all cases. Standard post-operative mobilisation and rehabilitation protocols were followed and patients were routinely called for clinico-radiological assessment at one month, three month, six month, 12 month and yearly thereafter. At a follow-up period of one year, functional outcome scores, Western Ontario and McMaster Universities Arthritis Index (WOMAC) and Knee Society Score (KSS) were calculated by an independent blinded observer (physiotherapy resident) to record pain, impairment of daily activities, satisfaction rate and expectations post TKA.
Post-operative joint line parameters (MEJL, LEJL, and FHJL) were re-calculated by the same observers using the same method (to avoid inter and intra observer variability) and joint line alteration from the preoperative values were calculated (Fig. 2). The post-operative limb alignment was assessed on a full-length X-ray scannogram to know the hip-knee-ankle axis(neutral/varus/valgus).
Fig. 2Post-operative assessment of joint line position with respect to anatomical bony landmarks (medial epicondyle, lateral epicondyle and fibula head) on a standing AP radiograph.
The statistical tools included quantitative data(mean and standard deviation) and qualitative data(frequency and percentage charts). Comparison among the study groups was done by student's unpaired “t” test. Association between the study groups was assessed by Fisher test, student-'t' test and Chi-Square test. The ‘p' value less than 0.05 was considered as a statistically significant outcome. Pearson's correlation test was used to evaluate the correlation between joint line elevation and outcome scores. Appropriate statistical software such as MS Excel and SPSS ver. 20 were used. MS Excel 2010 was used for graphical representation.
3. Results
The demographic data of the patients included in the study is shown in Table 1. The study was conducted on 120 patients which include 74 females (61.7%) and 46 males (38.3%). The mean age of patients was 67.37 (±14.38) years and the majority of cases (50%) were from the age group of 71–90 years. The right knee was affected in majority of the patients: 72/120 (60%) while the left knee was affected in 48/120 (40%) patients. Joint line elevation was observed in 104 out of 120 (86.7%) patients and 16/120 (13.3%) patients had no joint line elevation. In our series, the mean joint line elevation was 3.00 (±2.13) mm. We found no case in our series wherein the joint line was depressed. The mean post-op one-year Knee Society Score (KSS) of patients was 51.08 (±8.25). The mean post-op one-year WOMAC Score of patients was 76.62 (±7.47). We calculated the mean values for the post-op KSS and WOMAC scores for every one mm change in joint line position (Table 2). Our study highlighted a rising trend in WOMAC score and a declining trend in KSS score with every one mm rise in the joint line elevation. As high WOMAC scores and low KSS scores implicate poor outcome, our findings reiterate the fact that elevated joint line results in poor post-operative functional outcome after TKA. Post-operative assessment of hip-knee-ankle axis showed a neutral alignment in 104 patients and valgus alignment of 3–5° in 16 patients.
Table 1Demographic data of patients enrolled in the study.
Table 2Association of joint line elevation and post-operative functional scores showing a falling trend in KSS scores and rising trend in WOMAC scores with each 1 mm rise in joint line elevation.
Joint Line Alteration
Functional knee scores at follow up period of 1 year
It was found that there is a distinct variation between the outcomes of patients where joint line was elevated <5 mm to those where it was ≥ 5 mm. Hence the patients were sub-classified into two groups: Group A- Patients with joint line elevation <5 mm and Group B- Patients with joint line elevation ≥ 5 mm. Group A comprised of 98 patients which include 58 females (59.2%) and 40 males (40.8%). The mean pre-operative Range of Motion (ROM) was 85.20° (±10.47⁰) which improved to a mean ROM of 136.1° (±15.2⁰) post-operatively. The post-operative mechanical axis was neutral in 86 patients and valgus in 12 patients. Group B comprised of 22 patients which include 16 females (72.7%) and 6 males (27.3%). The mean pre-operative ROM was 89.70° (±8.97⁰) which improved to a mean ROM of 132.1° (±10.3⁰) post-operatvely. The post-operative mechanical axis was neutral in 18 patients and valgus in 4 patients. The difference in the two groups in terms of ROM and mechanical alignment were not statistically significant and variables such as age of the patient and pre-operative ROM were matched for both the groups.
The mean post-op one-year KSS score was significantly higher in patients with joint line elevation <5 mm (Group A) compared to joint line elevation ≥5 mm (Group B) {52.82 ± 7.564 vs. 40.73 ± 7.146; p < 0.001}. The mean post-op one-year WOMAC score was lower in patients with joint line elevation <5 mm (Group A) compared to joint line elevation ≥5 mm (Group B){65.51 ± 14.762 vs. 75.64 ± 8.203} and the difference was statistically significant as per Student t-test (p = 0.001) (Table 3).
Table 3Post-operative KSS and WOMAC scores demonstrate better functional outcome in patients with joint line elevation <5 mm (Group A) compared to those with joint line elevation (≥5 mm).
The most important finding of this study was that elevation of joint line has a deleterious effect on outcome score after primary TKA. Restoration of original anatomy and functional range is the primary objective in primary TKA. Joint line restoration prevents post-operative mid-flexion instability and anterior knee pain. Joint line elevation changes the centre of rotation of the knee, reduces the posterior condylar offset and changes the isometry of medial collateral ligament producing instability which is characteristically evident in mid-flexion.
Joint line restoration is crucial as many implant designs are guided by stability at the limits of flexion and extension alone. In a cadaveric study, Martin and Whiteside have demonstrated that even 5 mm anterior displacement of the joint line can produce mid-flexion instability of the knee.
Elevation of joint line increases the joint contact pressure in both the tibiofemoral and patellofemoral joint. The elevated joint line produces a pseudo patella baja and thus, the resurfaced patella is at a higher risk of wear and early failure.
In primary TKA, restoration of joint line is relatively easier as all bony and ligamentous references are preserved. This task is more challenging in revision cases as few of the bony landmarks are absent. Several bony landmarks (the medial epicondyle, the lateral epicondyle, the adductor tubercle and the fibular head) can be used intra-operatively to ascertain the restoration of joint line. Luyckx et al. defined “Adductor ratio”, the distance from adductor tubercle to the joint line divided by the femoral width as an accurate tool to determine joint line position.
This ratio is independent of size and is a more constant value. Also, palpation of the adductor tubercle intra-operatively is easier and more dependable. The use of medial and lateral epicondyles is also a familiar practice amongst many surgeons and the inter-epicondylar axis is a fairly constant landmark for intraoperative assessment of component positioning and joint line position.
The extent of acceptable range for joint line elevation post TKA is questionable. In a review of 116 knees, Figgie et al. showed that elevation of joint line of 8 mm or less was associated with good functional outcome, reduced pain, improved ROM and less revision rate as compared to those with more than 8 mm of joint line elevation.
Partington et al. reported an average KSS score of 141 in patients with less than 8 mm elevation as against an average score of 125 in patients with more than 8 mm joint line elevation after revision TKA.
Hassaballa et al. reviewed 100 cemented TKA and concluded that restoration of joint line within ±5 mm of the pre-operative measurement had a positive impact on post-operative ROM (116° for restored joint line versus 108° for elevated joint line) and kneeling ability.
Porteous et al. showed that restoration of joint line in revision TKA within 5 mm gives a significantly better outcome than in cases with elevation >5 mm. They recommended the use of distal femoral augments to achieve this goal.
Lieshout et al. published a systematic review of 27 articles and reiterated that a negative correlation exists between joint and functional outcome post TKA. They concluded that joint line elevation >4 mm can alter knee joint kinematics, resulting in poor outcome.
Our findings are also in accordance with these studies and re-establish the negative impact of joint line elevation on functional outcome after TKA. From our results, we propose a cut-off limit of ≤ 5 mm of joint line elevation to produce good functional outcome.
The effect of joint line elevation on the outcome in knee arthroplasty is discussed in literature in a controversial manner. Babazadeh et al. reported similar results in terms of functional outcome and joint line alteration between conventional and computer-assisted TKA.
Joint line positioncorrelates with function after primary total knee replacement: a randomised controlledtrial comparing conventional and computer-assisted surgery.
They reported no association between joint line elevation more than 2 mm and functional scores. Yang et al. reported that post-operative alterations in joint line in the range of −1 to +5 mm have no effect on functional outcomes.
Since the basic foundation of TKA is based on soft tissue balancing, appropriate bone cuts, cementing and joint line restoration, it will be prudent to state that aiming to restore the joint line as close to the pre-operative value as possible should be the aim of the operating surgeon to achieve the optimum outcome.
Certain intra-operative surgical principles should be taken into consideration to prevent severe alterations in joint line. The distal femoral bone cut should be minimised especially in cases with pre-operative bone loss. It is prudent to remove all posterior osteophytes before re-cutting the distal femur as these osteophytes tend to tent the posterior capsule and thus cause a reduction in extension space. It is necessary to choose the femoral component size based on the remaining anteroposterior bone stock rather than the mediolateral dimension. The femoral component needs to be positioned as posterior as possible without anterior notching to keep the flexion space as small as possible. It is advisable to systematically use distal and posterior femoral augments in cases of bone loss even in primary TKA, thereby restoring joint line position and posterior condylar offset. Appropriate pre-operative planning radiographs including X-rays of the contralateral knee can help the surgeon to determine the exact joint line distance from various bony landmarks before surgery.
There are few limitations of the study. First, the sample size is small. Second, the follow up period of one year is short to comment on long-term functional outcome.
5. Conclusions
Joint line elevation is a common occurrence and has a negative impact on post-operative functional outcomes in primary TKA. We conclude that the elevation of joint line ≥5 mm from the pre-operative value results in poor outcome scores than in cases with less than 5 mm raised joint line. Appropriate preoperative and intraoperative assessment of joint line distance from various bony landmarks will be helpful for the surgeon to achieve optimum restoration of the joint line. It is advisable to use distal femoral augments in cases with bone loss to prevent proximalization of the femoral component and associated complications.
Informed consent
Written informed consent was obtained from all individual participants included in the study.
Institutional Ethics Committee
Approval taken by Institutional Review Board and Departmental Review Board (IRB approval No: IEC/29/2019).
Author contributions
Dr. Spandan Kishore: Data acquisition.
Dr. Shubhranshu S. Mohanty: Supervision and validation.
Dr. Swapnil Anil Keny: Formal analysis.
Dr. Abhishek Kumar Rai: Writing original draft, review and editing.
Dr. Tushar Narayan Rathod: Methodology.
Dr. Prashant Kamble: Conceptualization.
Funding
This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors.
Declaration of competing interest
None.
Acknowledgement
None.
References
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Thornhill T.S.
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Wright J.
Katz J.N.
The dramatic increase in total kneereplacement utilization rates in the United States cannot be fully explained by growthin population size and the obesity epidemic.
Joint line positioncorrelates with function after primary total knee replacement: a randomised controlledtrial comparing conventional and computer-assisted surgery.
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