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  • Review
  • Open Access

Does malalignment affect patient reported outcomes following total knee arthroplasty: a systematic review of the literature

  • 1Email author,
  • 1,
  • 1,
  • 1,
  • 2 and
  • 1
SpringerPlus20165:1201

https://doi.org/10.1186/s40064-016-2790-4

  • Received: 9 December 2015
  • Accepted: 7 July 2016
  • Published:

Abstract

Background

Total knee replacement is an effective treatment for knee arthritis. While the majority of TKAs have demonstrated promising long-term results, up to 20 % of patients remain dissatisfied with the outcome of surgery at 1 year. Implant malalignment has been implicated as a contributing factor to less successful outcomes. Recent evidence has challenged the relationship between alignment and patient reported outcome measures. Given the number of procedures per year, clarity on this integral aspect of the procedure is necessary.

Objective

To investigate the association between malalignment and PROMS following primary TKA.

Methods

A systematic review of MEDLINE, CINHAL, and EMBASE was carried out to identify studies published from 2000 onwards. The study protocol including search strategy can be found on the PROSPERO database for systematic reviews.

Results

From a total of 2107 citations, 18 studies fulfilled the inclusion criteria, comprising of 2214 patients. Overall 41 comparisons were made between a malalignment parameter and a PROM, with 30 comparisons (73 %) demonstrating no association. However, 50 % (n = 9) of the studies with ‘Low risk’ radiological assessment methods have reported a statistically significant association between one or more parameter of malalignment and PROMS.

Conculsion

When considering malalignment in an individual parameter, there is an inconsistent relationship with PROMs scores. Malalignment may be related to worse PROMs scores, but if that relationship exists it is weak and of dubious clinical significance. However, this evidence is subject to limitations mainly related to the methods of assessing alignment post operatively and by the possibility that the premise of traditional mechanical alignment is erroneous. Larger longitudinal studies with a standardised, timely, and robust method for assessing alignment outcomes are required.

Keywords

  • Femoral Component
  • Total Knee Replacement
  • Mechanical Axis
  • Knee Society Score
  • Patient Report Outcome Measure

Background

Total knee Replacement (TKR) is considered an effective treatment for knee arthritis (Callahan et al. 1994). Over 77,000 TKA operations were performed during 2013 in England and Wales (Registry 2013) with expectations of increasing demand (Kane et al. 2003). While the majority of TKAs demonstrate significantly improved pain relief and function results (van Essen et al. 1998; March et al. 1999; Anderson et al. 1996), up to 20 % of patients remain unsatisfied with the outcome of surgery at 1 year (Kim et al. 2009; Scott et al. 2010; Baker et al. 2007; Robertsson et al. 2000; Bourne et al. 2010).

To ensure optimisation, an important technical objective during surgery is to achieve a perfect tri-planar component alignment (Sikorski 2008) with a neutrally aligned limb and a mechanical axis of 180° ± 3° and no tibio-femoral rotational mismatch (Ritter et al. 1994; Nicoll and Rowley 2010; Moreland 1988; Longstaff et al. 2009; Werner et al. 2005; Lotke and Ecker 1977; Bargren et al. 1983; Tew and Waugh 1985).

Three reasons to challenge the view that alignment in total knee replacements is of paramount importance have emerged (Eckhoff et al. 2005). Firstly, it is suggested that the evidence of poor outcomes secondary to malalignment is largely historic, based on studies of inferior implant designs (Bach et al. 2009; Bonner et al. 2011; Matziolis et al. 2010; Parratte et al. 2010), and the use of poor radiological techniques when assessing malalignment (Lotke and Ecker 1977). Secondly, outcomes following computer assisted TKA, proven to achieve better target alignment in comparison to conventional techniques, have demonstrated little evidence of clinical advantage (Matziolis et al. 2010; Cheng et al. 2012a). Thirdly, the choice of target for ideal alignment has been challenged by proponents of kinematically aligned TKA who have reported promising results (Howell et al. 2013a, b). Kinematic alignment aims to place the femoral component so that its transverse axis coincides with the primary transverse axis in the femur about which the tibia flexes and extends. As this axis is centred on the posterior condyles of the femur, which is not parallel to any standard coronal, sagittal or axial view, it is not measurable by standard means. With the removal of osteophytes the original ligament balance can be restored and the tibial component is placed with a longitudinal axis perpendicular to the transverse axis in the femur.

We performed a systematic review of the literature to answer the following research question: In patients undergoing primary total condylar knee replacement is malalignment, assessed radiologically, associated with functional outcomes and/or PROMs.

Methods

This review followed the guidelines described by the Agency for Healthcare Research and Quality (AHRQ) criteria (Viswanathan et al. 2008a). The review has been registered and published on the PROSPERO database; Protocol Number 2012:CRD42012001914 (Hadi et al. 2012).

Literature search

A literature search of the following databases was carried out: Medical Literature Analysis and Retrieval System Online, Bethesda, Maryland, USA (MEDLINE), Cumulative Index to Nursing and Allied Health Literature, Glendale, California USA (CINHAL), Excerpta Medica Database, Amsterdam, the Netherlands (EMBASE). A broad search strategy using MeSH terms “knee”, “replacement”, “alignment” and “outcome” was adopted. This was intended to identify English-language studies published from 2000 through to 2014. The search was restricted to this period to avoid the inclusion of studies with potentially poor implant designs and weak radiological assessment methods. The search was last performed on September 2014.

Eligibility criteria

Both observational and experimental designs were considered for inclusion in this review.

Inclusion criteria:
  • All patients who were deemed eligible for a primary TKA were considered.

  • All open procedures that used a total condylar knee replacement.

  • All described approaches.

  • All radiological alignment assessment methods and parameters described.

Exclusion criteria:
  • Studies that have fulfilled the inclusion criteria but have not provided adequate or clear information on the correlation analysis between malalignment and PROMs.

  • Studies with a mean follow-up of <6 months,

  • Abstract-only publications, expert opinions and chapters from books.

Extraction of data

Two investigators (MH, TB) independently reviewed the titles and abstracts to identify and retrieve all articles relevant to our research questions, disagreements were settled by consensus between the two reviewers or with a third investigator (MD).

The parameters of malalignment are illustrated in Fig. 1. For the purposes of this review we describe coronal alignment as the mechanical alignment, and describe the method of assessment (long leg or short leg radiograph).
Fig. 1
Fig. 1

A diagrammatic representation of different alignment parameters based on The Knee Society Total Knee Arthroplasty Roentgenographic Evaluation and Scoring System (Viswanathan et al. 2008a). The Coronal Tibiofemoral mechanical angle is the angle resulting from drawing a line from the centre of the femoral head down to centre of the ankle through the centre of the knee a ideally 180°. The coronal femoral angle cFA b ideally 96°—and coronal tibial angle cTA, c ideally 90°—are the angles between the components’ coronal axes (the line connecting the femoral components most distal condyles and the line along the horizontal tibial plate) and the bones’ coronal anatoical axes (line which bisects the medullary canal of the femur and tibia respectively). The coronal tibiofemoral anatomical angle is a combination of the coronal anatomical femoral axis and coronal anatomical tibial axis. The sagittal femoral sFA, d ideally 90°—and sagittal tibial sTA, e ideally between 83° and 90°—angles are the angles between the components’ sagittal axes (horizontal line perpendicular to the femoral component peg and line along the horizontal tibial plate) and the anatomical sagittal bones’ axes (line which bisects the medullary canal of the femur and tibia respectively). The axial femoral (aFA) f ideally 0°—and axial tibial—ideally within 15°—(aTA), g angles are the angles between the components’ axial axes (line through the centre of the femoral pegs and the line through the most posterior points of the tibial plate on axial views respectively) and the bones’ axial axes (surgical epicondylar femoral axis and the tibial tuberosity axis respectively). The combined components axial (aCRA) rotational alignment angles—ideally 0°—is the angle between the components axial axes

Quality assessments of included studies

All studies were assessed for their methodological qualities in accordance with their study design. Case control and Cohort studies were assessed using the Ottawa–Newcastle score system (Stang 2010). RCTs and Case series were assessed using an AHRQ design-specific scales (Viswanathan et al. 2008b).

Studies were further evaluated based on the quality of their radiological methods for assessing alignment. The evaluation was done using a five-question checklist devised for this review; the Radiological Assessment Quality (RAQ) criteria (Hadi et al. 2015). The items in the checklist together with their corresponding justification are described in the Additional file 1. Studies were deemed as low, unclear or high risk of assessment bias based on the radiological methods described. Sensitivity analysis using radiological assessment quality did not alter the results.

Statistical analysis

Due the exploratory nature of the research question, the summary of data was focused on descriptive statistics and qualitative assessment of the content of the identified literature. Meta-analysis was not part of the study protocol and was not conducted as the outcome measures, measure of alignment, and methods of assessment were heterogeneous. Given these limitations, it was thought it would produce a precise, but potentially spurious result (Egger et al. 1998).

Results

Search results

The initial search returned 2107 citations, of which 1719 were considered for screening. Details of the study selection process are described in Fig. 2.
Fig. 2
Fig. 2

PRISMA flow diagram including the details of our search results for this review. Figure shows the reasons behind study exclusion at each stage of the search and the number of studies identified at each point of the search

A total of 18 studies (Nicoll and Rowley 2010; Longstaff et al. 2009; Bach et al. 2009; Matziolis et al. 2010; Howell et al. 2013b; Aglietti et al. 2007; Bankes et al. 2003; Barrack et al. 2001; Bell et al. 2014; Blakeney et al. 2014; Choong et al. 2009; Czurda et al. 2010; Gothesen et al. 2014; Huang et al. 2012; Lutzner et al. 2010; Magnussen et al. 2011; Rienmuller et al. 2012; Stulberg et al. 2008) fulfilled the review inclusion criteria, including five RCTs (Blakeney et al. 2014; Choong et al. 2009; Gothesen et al. 2014; Huang et al. 2012; Lutzner et al. 2010), seven case control studies (Nicoll and Rowley 2010; Matziolis et al. 2010; Barrack et al. 2001; Bell et al. 2014; Czurda et al. 2010; Magnussen et al. 2011; Stulberg et al. 2008) and 6 case series (Longstaff et al. 2009; Bach et al. 2009; Howell et al. 2013b; Aglietti et al. 2007; Bankes et al. 2003; Rienmuller et al. 2012). The methodological quality-assessment of included studies is presented in Tables 1, 2 and 3. The results did not alter with subgroup analysis based on quality assessment.
Table 1

Quality assessment criteria for RCTs

Authors

Quality assessment

Judgment on risk of bias

Was the allocation sequence generated adequately?

Was the allocation of treatment adequately concealed

Did researchers rule out any unintended exposure that might bias results?

Were participants analysed within the groups they were originally assigned to?

Was the length of follow-up different between the groups

Were the outcome assessors blinded to the intervention or exposure status of participants?

Were the potential outcomes pre-specified by the researchers? Are all pre-specified outcomes reported?

If attrition was a concern were missing data handled appropriately?

Were outcomes assessed using valid and reliable measures across all study participants?

Blakeney et al. (2014)

Yes

No

Yes

Yes

No

No

Yes

Yes

Yes

Low risk

Choong et al. (2012)

Yes

No

Yes

Yes

No

No

Yes

Yes

Yes

Low risk

Huang et al. (2012)

Yes

No

Yes

Yes

No

No

Yes

Yes

Yes

Low risk

Lutzner et al. (2010)

Yes

No

Yes

Yes

No

No

Yes

Yes

Yes

Low risk

Gothesen et al. (2014)

Yes

Yes

Yes

Yes

No

Yes

Yes

Yes

Yes

Low risk

Assessed using AHRQ design specific scale (Stang 2010)

Table 2

Quality assessment of Case control and Cohort studies

Author

Quality assessment of case control studies

Is the case definition adequate?

Representativeness of the cases

Selection of controls

Definition of controls

Comparability of cases and controls on basis of design or analysis

Ascertainment of exposure

Same method of ascertainment for cases and controls

Non-Response rate

Total Newcastle Ottawa Scale (possible 9 starts)

Barrack et al. (2001)

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

8*

Bell et al. (2014)

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

8*

Czurda et al. (2010)

Yes

Yes

Yes

Yes

Yes

No

Yes

Yes

7*

Magnussen et al. (2011)

Yes

Yes

Yes

Yes

Yes

No

Yes

Yes

7*

Matziolis et al. (2010)

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

8*

Nicoll and Rowley (2010)

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

8*

Stulberg et al. (2008)

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

8*

Assessed using the Ottawa-Newcastle score (Viswanathan et al. 2008a)

* Represents how many stars were achieved in the assessment of quality for each study

Table 3

Quality assessment of Case series studies

Author

Quality assessment of case series

Consecutive selection of patients?

Were outcomes measured in an objective way?

Were confounders identified and controlled?

Was follow up sufficiently long and complete

Aglietti et al. (2007)

Yes

?

No

Yes

Bach et al. (2009)

Yes

?

No

Yes

Bankes et al. (2003)

Yes

Yes

No

Yes

Howell et al. (2013b)

?

Yes

Yes

Yes

Longstaff et al. (2009)

Yes

Yes

Yes

Yes

Rienmüller et al. (2012)

Yes

?

No

Yes

Assessed using AHRQ design specific scale (Stang 2010)

The total number of patients recruited in all included studies was 2214 patients. Minimal patient baseline characteristics were reported, however, where reported they were comparable between studies. Study characteristics can be seen in Table 4.
Table 4

Study characteristics of included studies in this review

Author and journal

Study design

Sample size

Follow up (mean range)

Number of patients lost to follow up

Final study sample size

Choong et al. (2012) J Athro

RCT (single centre)

120

1 year

9

111

Lutzner et al. (2010) Knee Surg. Sports Trauma Arthos

RCT (single centre)

80

1.8 years

7

73

Huang et al. (2012) Journal of Arthoplasty

RCT (single centre)

111

5 years

21

90

Blakeney et al. (2014) The Knee

RCT (single centre)

107

46 months

14

93

Gothesen et al. (2014) JBJS

RCT (multi-centre)

194

5 years

19

175

Barrack et al. (2001) CORR

Case control (single centre)

30

5.7 years

2

28

Stulberg et al. (2008) Orthopaedics

Case control (single centre)

58

2.5 years

6

52

Nicoll and Rowley (2010) JBJS

Case control (single centre)

61

>1 year

23

39

Matziolis et al. (2010) Arch Orthop Trauma Surg

Case control (single centre)

218 (from a database)

5–10 years

168

50

Czurda et al. (2010) Knee Surg Sport Trau Arthrosc

Case control (single centre)

38

2.2 years

0

38

Magnussen et al. (2011) CORR

Case control (single centre)

608

Median 4.7 years (2–19.8)

55

553

Bell et al. (2014) The Knee

Case control (single centre)

127

1 year

15

112

Bankes et al. (2003) The Knee

Case series (single centre)

198

6.5 years

0

198

Aglietti et al. (2007) CORR

Case series (single centre)

64

8 Years

19

53

Longstaff et al. (2009) J Arthro

Case series (single centre)

159

1 year

9

146

Bach et al. (2009) The Knee

Case series (single centre)

105

10.8 years

7

98

Rienmüller et al. (2012) International Orthopaedics

Case series (single centre)

219

5 Years

15

204

Howell et al. (2013b) Knee Surg Sport Trau Arthrosc

Case series (single centre)

101

6–9 months

1

101

The functional and PROMS outcomes identified in this review included: Knee Society Score (KSS), Hospital for Special Surgery Score (HSS), Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), SF-12, SF-36, EuroQol, patella-femoral symptoms Score, Bristol score, Nottingham health profile, Visual analogue scale (VAS).

Out of the possible malalignment parameters considering the component’s six degrees of freedom, ten parameters were reported. Multiple different measures were used to measure coronal alignment, with varying nomenclature. Sagittal and axial alignments of the tibial and femoral components were not subject to this confusing nomenclature. See the Additional file 1 for a full description of each alignment parameter with detailed findings from each paper.

Quality assessment

Tables 1 and 2 demonstrate the quality assessment of each included study.

Radiological assessment

Table 5 demonstrates the radiological characteristics of each study using the RAQ criteria.
Table 5

Radiological methods quality assessment of included studies

Author

Modality of image

Timing of image

Weight bearing

Protocol/standardisation

Rater reliability assessment

Outcome

Choong et al. (2012)

CT, LLR

6 weeks

Y

Y

N

Low risk

Lutzner et al. (2010)

CT, LLR

18–32 months

Y

U

N

High risk

Huang et al. (2012)

CT, LLR

6 weeks

Y

Y

N

Low risk

Blakeney et al. (2014)

CT (3D)

3 months

N

Y

N

Medium risk

Gothesen et al. (2014)

CT, LLR

3 months

Y

Y

N

Low risk

Barrack et al. (2001)

CT, LLR

At latest follow up

Y

U

N

High risk

Stulberg et al. (2008)

LLR, SLR, Navigation system

4 weeks and 2 years

Y

Y

N

Low risk

Nicoll and Rowley (2010) JBJS

CT, SLR

At least 1 year after TKR

N

U

N Senior author

High risk

Matziolis et al. (2010)

LLR

Latest follow up

Y

Y

Y

High risk

Czurda et al. (2010)

CT, LLR

At 1st follow up

Y

Y

N Independent radiologist

Low risk

Magnussen et al. (2011)

LLR

Follow up (varied)

Y

Y

Y

High risk

Bell et al. (2014)

CT

26 months

N

U

MSK radiologist

High risk

Bankes et al. (2003)

SLR

3 and 12 month follow up

Y

Y

N

Low risk

Aglietti et al. (2007)

LLR

Latest follow up

Y

Stress to assess varus-valgus stability

N

High risk

Longstaff et al. (2009)

CT

6 months

N

Y

Y

Low risk

Bach et al. (2009)

SLR

At follow up

N

Y

N Experienced radiologist

High risk

Rienmüller et al. (2012)

LLR, Axial XR

5 years

N

Y

Y

High risk

Howell et al. (2013b)

CT

2 days

N

Y

N

Medium risk

We devised a 5 point checklist (Fig. 2) and all studies were assessed using this checklist to identify whether they were high/low risk

CT computerised tomography, LLR long leg radiograph, SLR short leg radiograph, Y yes, N no, U unknown

Association between malalignment and patient reported outcome measures (PROMs)

Overall 41 comparisons were made between a malalignment parameter and a PROM, with 30 comparisons (73 %) demonstrating no association. Of the 18 studies, 12 studies (67 %) demonstrated an association between malalignment in one or more parameter of alignment and a worse patient reported outcome. Of these, nine studies (50 %) applied radiological methods with a low or medium risk of bias.

We summarised the association between malalignment and PROMs according to the plane of assessment and the individual components.

In the coronal plane, five out (Longstaff et al. 2009; Cheng et al. 2012a; Aglietti et al. 2007; Blakeney et al. 2014; Huang et al. 2012) of fourteen studies (Nicoll and Rowley 2010; Longstaff et al. 2009; Bach et al. 2009; Matziolis et al. 2010; Cheng et al. 2012a; Howell et al. 2013b; Aglietti et al. 2007; Bankes et al. 2003; Blakeney et al. 2014; Czurda et al. 2010; Gothesen et al. 2014; Huang et al. 2012; Magnussen et al. 2011; Stulberg et al. 2008) showed an association between malalignment in the coronal plane and worse PROM scores (Table 6; Fig. 3) provide graphical representation of this.
Table 6

Association between coronal malalignment and worse outcome

Author

Sample size

Type of radiograph

RAQ score

Outcome measure

Malalignment parameter

Association between malalignment and worse outcome

Aglietti et al. (2007)

53

LL

High risk

KSS (Clinical) HHS Patella score

cTFmA

Yes

Choong et al. (2012)

111

LL

Low risk

IKS SF-12

cTFmA

Yes

Blakeney et al. (2014)

93

CT

Medium risk

SF-12 OKS

cTFmA

Yes

Huang et al. (2012)

90

LL

Low risk

IKS SF-12

cTFmA

Yes

Longstaff et al. (2009)

146

CT

Low risk

KSS

cTA, cFA

Yes

Howell et al. (2013b)

101

CT

Medium risk

OKS WOMAC

cTFmA, cTA

No

Magnussen et al. (2011)

553

LL

High risk

KSS

cTFmA, cTA, cFA

No

Matziolis et al. (2010)

50

LL

High risk

KSS WOMAC SF36KSS

cTFmA, cTA, cFA

No

Stulberg et al. (2008)

52

LL

Low risk

KSS

cTFmA

No

Gothesen et al. (2014)

175

LL

Low risk

KSS

cTFmA, cTA, cFA

No

Czurda et al. (2010)

38

LL

Low risk

WOMAC KSS

cTFmA, cFA

No

Bach et al. (2009)

98

SL

High risk

KSS, HSS, Bristol score, NHP

cTFaA, cTA, cFA

No

Bankes et al. (2003)

198

SL

Low risk

KSS

cTFaA, cTA, cFA

No

Nicoll and Rowley (2010)

45

SL

High risk

KSS

cTFaA, cTA, cFA

No

KSS knee society score, HHS harris hip score, NHP Nottingham health profile, WOMAC Western Ontario and McMaster Universities Arthritis index, OKS Oxford knee score, SF-12 short form-12, cTFmA coronal tibio-femoral mechanical alignment, cTFaA coronal tibio-femoral anatomical alignment, cTA coronal tibial alignment, cFA coronal femoral alignment, LL long leg radiograph, SL straight leg radiograph

Fig. 3
Fig. 3

Graph demonstrating the number of studies demonstrating an association between malalignment and worse PROMs (patient reported outcome measures) scores based on imaging in the coronal, sagittal and axial view

Only four studies (Longstaff et al. 2009; Bach et al. 2009; Bankes et al. 2003; Stulberg et al. 2008) investigated sagittal malalignment and its relationship with PROM, with none of these demonstrating a statistically significant association between femoral or tibial malalignment and worse outcomes (Table 7; Fig. 3).
Table 7

Association between sagittal malalignment and worse outcome

Author

Sample size

Type of radiograph

RAQ score

Outcome measure

Malalignment parameter

Association between malalignment and worse outcome

Bankes et al. (2003)

198

SL

Low risk

KSS

sFA, sTA

No

Bach et al. (2009)

98

SL

High risk

KSS, HSS, Bristol score, NHP

sFA, sTA

No

Stulberg et al. (2008)

52

LL

Low risk

KSS

sFA, sTA

No

Longstaff et al. (2009)

146

CT

Low risk

KSS

sFA, sTA

No

KSS knee society score, HSS hospital for special surgery score, NHP Nottingham health profile, sTA sagittal tibial angle, sFA sagittal femoral angle, LL long leg radiograph, SL straight leg radiograph

Four (Barrack et al. 2001; Bell et al. 2014; Czurda et al. 2010; Lutzner et al. 2010) out of eight studies (Nicoll and Rowley 2010; Longstaff et al. 2009; Howell et al. 2013b; Barrack et al. 2001; Bell et al. 2014; Czurda et al. 2010; Lutzner et al. 2010; Rienmuller et al. 2012) found a statistically significant association between malalignment in the axial view and worsening patient reported outcome measures (Table 8; Fig. 3).
Table 8

Association between axial malalignment and worse outcome

Author

Sample size

Type of radiograph

RAQ score

Outcome measure

Association between malalignment and worse outcome

Barrack et al. (2001)

28

CT, LLR

High risk

KSS

Yes

Bell et al. (2014)

112

CT

High risk

OKS VAS

Yes

Lutzner et al. (2010)

73

CT, LLR

High risk

KSS

Yes

Czurda et al. (2010)

38

CT, LLR

Low risk

WOMAC KSS

Yes

Rienmüller et al. (2012)

204

LLR, Axial XR

High risk

KSS

No

Howell et al. (2013b)

101

CT

Medium risk

OKS WOMAC

No

Nicoll and Rowley (2010)

45

CT, SLR

High risk

KSS

No

Longstaff et al. (2009)

146

CT

Low risk

KSS

No

KSS knee society score, WOMAC Western Ontario and McMaster Universities Arthritis Index, OKS Oxford knee score, VAS visual analogue score for pain, LL long leg radiograph, SL straight leg radiograph

Finally, Fig. 4 demonstrate the rate each individual malalignment parameter’s association with PROMS outcome. The chart highlights the number of studies with low and high risk for radiological assessment bias as per the RAQ criteria.
Fig. 4
Fig. 4

Graph demonstrating rate each malalignment parameter is reported to associate with outcome. Studies are divided into with low and high risk of radiological assessment bias as per he RAQ criteria. cTA coronal tibial angle, sTA sagittal tibial angle, cFA coronal femoral angle, sFA sagittal femoral angle, cTFmA coronal tibio-femoral mechanical angle, cTFaA coronal tibiofemoral anatomical angle, aFRA axial femoral rotational angle, aTRA axial tibial rotational angle, aCRA axial combined rotational/mismatch angle

Discussion

The main findings of this review were that 50 % (n = 9) of the studies with ‘Low risk’ radiological assessment methods have reported a statistically significant association between one or more parameter of malalignment and PROMS. Overall 41 comparisons were made between a malalignment parameter and outcome within the included studies, with only 11 comparisons (27 %) demonstrating an association. With a p value of 0.05, we would expect two of these associations by chance. This suggests that the effect of malalignment on PROMs is likely to be small and it is unclear from this review the clinical significance of this finding. When assessing each parameter individually:

Coronal malalignment

In the literature, coronal malalignment is seen regarded as one of the most important factors determining the long-term prosthesis survival. Several authors stressed the importance of restoring limb coronal mechanical alignment to within 180°. In this review, as many as 64 % of studies investigating alignment in the coronal plane showed no associated between malalignment and worse outcome measures.

Sagittal malalignment

Components malalignment on this plane can alter the posterior tibial slop and affect the flexion and extension gaps. This may result in overstuffing and limited joint range. Femoral notching can be seen in excessive femoral component extension position. However, 100 % of studies reviewed in this review showed no associated between sagittal malalignment and worse outcome measures.

Axial malalignment

Many references exist for measuring femoral and tibia component rotation. Individual component malalignment and combined mismatch can result in abnormal patella tracking and subsequent anterior knee pain. Our review show 50 % of studies found an association between malalignment and worse PROMs.

Strengths and limitations

Several caveats exist in interpreting this paper, mostly based on the limitations of the studies involved, and the complexity of the topic in general. There is a lack of consistency in the way different studies assessed alignment following a TKA. For example, the use of long leg and short leg radiographs. When using long leg radiographs a direct comparison between the mechanical axis and the femoral and tibial alignment can be made in the coronal plane. If using a short leg radiograph an indirect assessment is made, based on the assumption that the intramedullary canal of the femur deviates 6° from the mechanical axis. In reality this deviation is variable, making this assessment method less accurate. To address this, a RAQ checklist was devised to assess the radiological methods, although this did not alter the overall results of the review by sub-analysis. These variations in methodology (combined with variation in PROMS scores) make meta-analysis problematic.

Furthermore, a number of studies restricted their analysis to one or two parameters of alignment. This approach is problematic given the relative interconnection between the alignment components in a TKA (Berend et al. 2004; Ritter et al. 2011). Berend et al. (2004) found the effect of malalignment in one implant moderated by the alignment of the other. Ritter et al. (2011) concluded that correction of the alignment of the second component in order to produce an overall neutrally aligned knee replacement when the first component has been malaligned may increase the risk of failure. These findings suggest a complex interplay between all measures of alignment in both the tibial and the femoral components that cannot be simplified to conventional definitions of “malaligned” or “aligned”. Given that some studies did not report findings for certain parameters there is the potential for publication bias as studies where no relationship was found contained missing data.

In addition, the parameters of malalignment were poorly defined. Studies presented malalignment data either in terms of deviation from the leg axis in the arithmetic mean or as two groups of ‘Aligned’ versus’ Malaligned’ or ‘Outliers’. While the majority of studies applied a ±3° range around a perfect alignment measurement, some studies had a more stringent criterion applying a ±2° range. Applying this narrow range, Longstaff et al. (2009) found better functional outcomes with good coronal femoral alignment and only a trend to better function at 1 year on patients with ‘good’ coronal tibial and ‘good’ sagittal tibial and sagittal femoral alignment.

The characteristics of the patient-related clinical outcome measures used by the studies included in this review may have contributed to the quality of the evidence presented. Some quality of life outcomes can suffer from ceiling effects that can result in abolishing the advantage of perfect aligned implants in comparison to those with mild degree of malalignment. The KSS, which is a regularly used functional score and most commonly identified in this review is subject to assessor bias.

Seven studies in this review used CAS. This is relatively small given the popularity of this technique and its consistency at achieving better alignment (Cheng et al. 2012b; Fu et al. 2012; Hetaimish et al. 2012). It would be reasonable to assume that studies reporting CAS outcomes would provide data on the association between alignment and outcome. However, the literature suggest that CAS surgery studies are usually under powered for sub-analysis of aligned versus malaligned and therefore not reported (Khan et al. 2012). Eckhoff et al. assessed CT scans on 90 patients to investigate axial limb alignment. They found that normal individuals expressed a wide range in the straight-line mechanical axis. This has two consequences; if surgical correction of a pathological knee to achieve a straight mechanical axis does not return the mechanical alignment to normal. This can lead to increased pressures on the polyethylene components increasing wear rates. Secondly, if a knee is not straight, the procedures achieve mechanical alignment will alter soft tissue balance affecting PROMs scores. This study has important implications for CAS surgery, if the algorithms do not incorporate this wide variation in natural morphology and kinematics of the knee (as evidenced by Eckhoff) the end result of CAS surgery can lead to further malaligned knees, increased wear and worsening PROMs scores (Eckhoff et al. 2005).

When our results are viewed from the kinematic perspective the unclear association between mechanical alignment and outcome makes sense given that there is a large variation in the anatomy of femora and tibiae and that most patients do not have a neutral hip–knee–ankle axis (Hollister et al. 1993). It is entirely possible that an anatomically (kinematically) aligned, but mechanically malaligned, implanted prosthesis could recreate a patient’s preoperative kinematics. Howell et al. (2015) concluded that kinematic aligned knee replacement did not adversely affect implant survival or function as it restores the constitutional alignment of the limb and joint line, subsequently avoiding collateral ligament imbalances. This would create a group of patients that, for the purposes of the studies included in this review, were “malaligned”, but had good PROMs scores based on their alignment. This could explain the dubious relationship demonstrated between alignment and outcome.

In conclusion, alignment in an individual parameter may have a weak, and perhaps clinically insignificant, effect on scores. However, this evidence is subject to limitations mainly related to the methods of assessing alignment post operatively. Larger longitudinal studies with a standardised, timely, and robust method for assessing alignment outcomes are required.

Declarations

Authors’ contributions

MH, TB: Literature search, data extraction and production of manuscript. IA: Literature search, data extraction and production of figures and manuscript. MD: Senior author for literature search and production of manuscript. PM: Conception of idea and production of manuscript. DG: Production of manuscript and senior author. All authors read and approved the final manuscript.

Acknowledgements

None.

Competing interest

The authors declare that they have no competing interests.

Ethical approval

This article does not contain any studies with human participants or animals performed by any of the authors.

Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Authors’ Affiliations

(1)
Trauma and Orthopaedic Department, UHCW, University of Warwick, Coventry, CV2 2DX, UK
(2)
John Radcliffe Hospital, University of Oxford, Oxford, OX3 9DU, UK

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