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VOLUME 3, ISSUE 1 

 2020 

 

 

RESEARCH ARTICLE 

Köhler T.M, Bellmann M, Blumentritt S. Polycentric Exoprosthetic Knee Joints – Extent of Shortening during Swing Phase. Canadian Prosthetics & Orthotics 

Journal. 2020;Volume3, Issue1, No.5. https://doi.org/10.33137/cpoj.v3i1.33768 

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https://doi.org/10.33137/cpoj.v3i1.33768


 

1 

Köhler T.M, Bellmann M, Blumentritt S. Polycentric Exoprosthetic Knee Joints – Extent of Shortening during Swing Phase. Canadian Prosthetics & Orthotics 
Journal. 2020;Volume3, Issue1, No.5. https://doi.org/10.33137/cpoj.v3i1.33768 

 

 

 
RESEARCH ARTICLE 

 

POLYCENTRIC EXOPROSTHETIC KNEE JOINTS – EXTENT OF SHORTENING DURING SWING 
PHASE  

 
Köhler T.M1*, Bellmann M1, Blumentritt S2         
 

1 Clinical Research and Services, Research Biomechanics, Ottobock SE & Co. KGaA, Göttingen, Germany. 
2 Private University of Applied Science, Göttingen, Germany. 

 

 

 

 

  

 

 

 

 

 

 

 

 

 

INTRODUCTION   

Tripping is a safety risk for amputees and it is mainly 

affected by ground clearance during swing phase. In mid–

swing, the toe is at the minimum distance to the ground 

while the shank rotates forward.1,2 To compensate for a lack 

of ground clearance, transfemoral amputees often perform 

compensatory movements including plantar flexion of the 

intact ankle during prosthetic swing (vaulting), lifting the hip 

on the prosthetic side during prosthetic swing (hip hiking), 

and swinging the prosthesis forward in an arc by abducting 

the hip early in swing and then adducting the hip late in 

swing (circumduction). These gait abnormalities reduce 

walking efficiency and are therefore undesirable.1-4 During 

prosthetic alignment, the length of the prosthesis can be 

reduced when compared to the intact limb and this might 

result in increased ground clearance during walking. 

However, this intervention can lead to other compensatory 

movements,5 reduced walking efficiency6 and lower back 

pain.7 Therefore, a prosthesis that provides a technical 

solution for ensuring adequate ground clearance during 

swing phase on the prosthetic side is desirable. Sockets 

with vacuum suspension can minimize longitudinal 

movement (pistoning) between the socket and limb8 and 

thus reduce the functional elongation of the prosthesis. 

Prosthetic feet generating ankle dorsiflexion during swing 

phase showed more ground clearance compared to 

conventional energy storing and returning feet.9,10 

 
OPEN  ACCESS Volume 3, Issue 1, Article No.5. 2020 

 

 

Journal Homepage: https://jps.library.utoronto.ca/index.php/cpoj/index 

 

ABSTRACT 

BACKGROUND: An often assumed advantage of polycentric knee joints compared to monocentric ones 

is the improved ground clearance during swing phase due to the geometric shortening of the lower leg 

segment (LLS).  

OBJECTIVE: To investigate whether polycentric knee joints considerably improve ground clearance and 

to evaluate the influence of prosthetic alignment on the extent of ground clearance.  

METHODOLOGY: 11 polycentric and 2 monocentric knee joints were attached to a rigid, stationary 

testing device. Shortening of the LLS and the resulting ground clearance during knee flexion were 

measured. Prosthetic components were mounted at the same height and the anterior-posterior position 

was in accordance with the manufacturer's alignment recommendations.  

FINDINGS: Shortening of up to 14.7 (SD=0.0) mm at the instance of minimal ground clearance during 

swing phase was measured. One knee joint elongated by 4.4 (SD=0.0) mm. Measurements of the ground 

clearance demonstrated differences up to 25.4 (SD=0.0) mm. One monocentric knee joint provided more 

ground clearance when compared to 8 of the polycentric knee joints investigated.   

CONCLUSION: Only some polycentric knee joints shorten appreciably during swing phase. With an 

optimized prosthetic alignment and a well-designed swing phase control, a monocentric knee joint may 

generate greater ground clearance compared to a polycentric knee joint. 

ARTICLE INFO 

Received: February 14, 2020 

Accepted: July 20, 2020 

Published: July 29, 2020 

CITATION 

Köhler T.M, Bellmann M, 

Blumentritt S. Polycentric 

Exoprosthetic Knee Joints – 

Extent of Shortening during 

Swing Phase. Canadian 

Prosthetics & Orthotics 

Journal. 2020;Volume3, 

Issue1, No.5. 

https://doi.org/10.33137/cpoj.

v3i1.33768 

KEYWORDS 

Amputation, Ground 

Clearance, Knee Joint, 

Polycentric, Prosthesis, Limb 

Loss, Rehabilitation. 

 

* CORRESPONDING AUTHOR: 
Thomas Maximilian Köhler, MSc 
 

Ottobock: Hermann-Rein-Straße 2a, 37075, Göttingen. 

E-mail: thomasmaximilian.koehler@ottobock.de 

ORCID: https://orcid.org/0000-0002-5063-121X 

 

 

https://doi.org/10.33137/cpoj.v3i1.33768
https://jps.library.utoronto.ca/index.php/cpoj/index
https://doi.org/10.33137/cpoj.v3i1.33768
https://doi.org/10.33137/cpoj.v3i1.33768
file:///D:/CPOJ%20Submitted%20Articles/ACCEPTED-Completed/33-Thirty%20Three%20Article-CPOJ-4T-2020-Ottobook-Kohler/Article%20Proof/thomasmaximilian.koehler@ottobock.de


 

2 

Köhler T.M, Bellmann M, Blumentritt S. Polycentric Exoprosthetic Knee Joints – Extent of Shortening during Swing Phase. Canadian Prosthetics & Orthotics 
Journal. 2020;Volume3, Issue1, No.5. https://doi.org/10.33137/cpoj.v3i1.33768 

ISSN: 2561-987X SWING SHORTENING IN POLYCENTRIC KNEE JOINTS 

Köhler et al. 2020 

 
CPOJ 

 
Polycentric knee joints also showed more ground clearance 

during swing phase when compared to monocentric knee 

joints.1,11   

Due to their technical design, prosthetic knee components 

can be divided into two main categories: monocentric and 

polycentric knee joints. Monocentric knee joints have a 

single center of rotation (CR) that is independent of the knee 

flexion angle. In polycentric knee joints, the upper and lower 

parts of the knee joint are usually connected via a four-bar 

linkage mechanism. They rotate around an instantaneous 

center of rotation (ICR), which is dependent upon the knee 

flexion angle. The ICR results from the intersection of the 

longitudinal axes of the anterior and the posterior linkages. 

Depending on the specific linkage mechanism, the ICR is 

usually located outside of the knee joint construction 

itself.12Consequently, there is a high degree of stance phase 

safety provided at heel strike since a fully extended knee 

joint results in an ICR that is located posterior to the knee 

joint and this is posterior to the vector of the ground reaction 

force. For patients with knee disarticulation or long 

transfemoral residual limbs, polycentric knee joints are 

preferred from a cosmetic point of view when seated, due to 

the minimized protrusion at the distal end of the socket in a 

flexed knee position.11 As an additional advantage, the 

shortening of the shank during swing phase or virtual ankle 

dorsiflexion is often stated.1, 11, 13 

Due to their design, polycentric knee joints are able to 

generate greater ground clearance in a flexed knee position 

compared to monocentric ones based on this shortening 

effect. Hence, in a study from 1996, ground clearance 

values were 9-32 mm higher for polycentric joints.11 Another 

study reported an average of 22 mm more ground clearance 

for the investigated polycentric knee joints.1 Currently, there 

is a larger variation in the length and orientation of the 

linkages in polycentric knee joints than in the formerly 

reported studies. Consequently, for the knee joints 

investigated in this study shortening effects of less than the 

formerly reported 9 mm were expected. 

Objective 

The aim of this study was to investigate whether polycentric 

knee joints generally show a marked shortening of the lower 

leg segment during prosthetic swing phase resulting in a 

clear advantage for patients regarding tripping. 

Furthermore, the influence of prosthetic alignment with 

regards to ground clearance was also examined. 

METHODOLOGY 

Knee joints 

In this study, 11 polycentric knee joints were investigated: 

The 3R46/3R55, 3R60, 3R106 (Ottobock SE & Co. KGaA, 

Duderstadt, Germany), Total Knee 2000, OHP3/KHP3, 

OH5/KH5 (Össur, Reykjavik, Iceland), TGK-4P01P,  

TK-4P00S (Teh Lin, Taipei, Taiwan), JT22 (Uniprox, 

Zeulenroda, Germany), KX06 (Blatchford, Basingstoke, 

United Kingdom) and Allux (Nabtesco, Tokyo, Japan). The 

monocentric knee joints 3R45/3R95 and C-Leg (Ottobock 

SE & Co. KGaA, Duderstadt, Germany) were also 

examined. Except for the 3R60 and Total Knee 2000, each 

of the polycentric knee joints utilizes a 4-bar linkage system. 

The fifth axis of the 3R60 only affects stance phase. 

Therefore, the 3R60 is effectively a 4-bar linkage knee joint 

during swing phase. The Total Knee uses a 7 axis linkage 

mechanism and all of the axes are involved during swing 

phase motion (Figure 1). 

                    

Figure 1: Polycentric and monocentric knee joints investigated with schematic illustration of the ICR respectively (scaling is not uniform between 

the knee joints), *according to Van de Veen PG ,2001.14 

 

3R60    3R46/3R55      KX06         OHP3/KHP3  OH5/KH5   Total Knee*      JT22      3R106 

C-Leg 3    C-Leg 4          Allux      TGK-4P01P   TK-4P00S 

3R45/3R95  

https://doi.org/10.33137/cpoj.v3i1.33768


 

3 

Köhler T.M, Bellmann M, Blumentritt S. Polycentric Exoprosthetic Knee Joints – Extent of Shortening during Swing Phase. Canadian Prosthetics & Orthotics 
Journal. 2020;Volume3, Issue1, No.5. https://doi.org/10.33137/cpoj.v3i1.33768 

ISSN: 2561-987X SWING SHORTENING IN POLYCENTRIC KNEE JOINTS 

Köhler et al. 2020 

 
CPOJ 

 
Prosthetic alignment 

Bench alignment of the prosthesis was performed using a 

L.A.S.A.R assembly (Ottobock SE & Co. KGaA, Duderstadt, 

Germany). For each alignment, a Trias foot (Ottobock SE & 

Co. KGaA, Duderstadt, Germany) with a length of 260 mm 

and an effective heel height of 10 mm was used. The 7E7 

(Otto Bock, Duderstadt, Germany) was utilized as a hip 

joint. The knee joint was adjusted to a height of 520 mm and 

the 7E7 to 900 mm. The hip joint axis was located 10 mm 

anterior to the reference line since the physiological hip 

center of rotation is located approximately 10 mm anterior 

to the greater trochanter and this is typically used as the 

reference point for the prosthetic socket anterior-posterior 

position (Figure 2). The length of the prosthesis was 

approximated according to an average person (male, 1.80 

m) based on anthropomorphic data.  

The A-P (anterior-posterior) position of each knee 

component and foot were aligned according to the 

manufacturer’s instructions for the individual knee joint 

(Table 1). Following these recommendations, the 

alignments should provide realistic data for each specific 

knee joint investigated in this study.  The Allux and C-Leg 

were tested according to their recommended alignment. 

Subsequently, these knee joints were re-tested with the 

others respective recommended alignment to observe the 

effect of alignment differences on ground clearance  without 

the possible influence of shortening effects. 

 

Table 1. Alignment position of prosthetic knee joints investigated.  

Knee joint Position foot 
Position alignment 

reference point* [mm] 
anterior + / posterior  - 

Allux FP 1 / FP 2 0/0 

KX06 FP 1 0 

Total Knee FP 2 +11** 

OHP3/KHP3 FP 2 0 

OH5/KH5 FP 2 0 

TGK-4P01P FP 2 0 

TK-4P00S FP 2 0 

JT22 FP 1 -25 

3R46/3R55 FP 1 -10 

3R60 FP 1 0 

3R106 FP 1 0 

3R45/3R95 FP 1 -15 

C-Leg FP 1 / FP 1 / FP 2 +5 / 0 / 0 

 

*Upper anterior axis for polycentric knee joints, knee rotation axis 

for monocentric knee joints **Offset between reference axis defined 

by the manufacturer (aligned on the alignment reference line) and 

upper anterior axis is approximately 11 mm. 

Each foot was positioned as follows: 

• Foot Position 1 (FP 1): The middle of the foot is 

positioned 30 mm anterior to the alignment reference 

line 

• Foot Position 2 (FP 2): The alignment reference line 

divides the foot into 1/3 rear foot and 2/3 forefoot  

In order to ensure that the motion only occurred in the 

sagittal plane, the axes of rotation for the hip and knee as 

well as the distal surface of the foot were all aligned 

perpendicular to the sagittal plane for all test setups. 

Experimental setup 

The test prosthesis was connected via the proximal part of 

the 7E7 joint to a stationary device. This configuration 

allowed a step-less adjustment of the hip flexion angle. The 

position of the hip axis of rotation was stationary. The study 

from Winter cites an average hip angle of 23° at the instance 

of minimal ground clearance while walking at a self-

selected, medium walking speed.15 In this study, four hip 

flexion angles were evaluated: 15°, 20°, 25° and 30° (angle 

from vertical). This range should cover the potential hip 

flexion of a transfemoral amputee at the instance of 

minimum ground clearance during swing phase (Figure 3). 

Due to different pyramid adapter positions for each knee 

joint, the resulting inclination of the tube adapter between 

hip and knee joint varies during bench alignment. Starting 

from the individual inclinations of the tube adapter, the hip 

joint was flexed by the respective angles investigated.  This 

ensures identical effective hip flexion angles for all 

investigated knee joints (Figure 2). 

Measuring system 

Kinematic parameters were measured via an optoelectronic 

12-camera motion capture system at a sampling rate of 200 

Hz (Vicon Bonita B10, Vicon Peak, Oxford, United 

Kingdom).  Four retro-reflective markers were attached to 

the prosthesis: Hip joint axis, knee joint axis (Polycentric: 

upper anterior axis), lateral ankle adapter screw, big toe 

(hallux) (Figure 2). 3D trajectories (X,Y,Z) of the markers 

were captured with an accuracy of 0.5 mm.16 

Experimental procedure 

The specifically defined hip angle was adjusted prior to each 

measurement. The knee joint was flexed and extended 

manually from the fully extended position to a knee flexion 

angle of approximately 90° and then back into full extension 

six times during the measurement (six motion cycles).  

Data analysis 

Minimum ground clearance was defined as the event when 

the z-component of the big toe marker reached its minimum. 

Shortening of the lower leg segment was defined by the 

difference of the distance between the knee marker and the 

https://doi.org/10.33137/cpoj.v3i1.33768


 

4 

Köhler T.M, Bellmann M, Blumentritt S. Polycentric Exoprosthetic Knee Joints – Extent of Shortening during Swing Phase. Canadian Prosthetics & Orthotics 
Journal. 2020;Volume3, Issue1, No.5. https://doi.org/10.33137/cpoj.v3i1.33768 

ISSN: 2561-987X SWING SHORTENING IN POLYCENTRIC KNEE JOINTS 

Köhler et al. 2020 

 
CPOJ 

 
big toe marker at full knee extension, and at the instant of 

minimal ground clearance. The distances were calculated 

within the sagittal plane, via the XZ- coordinates (Figure 2). 

This definition was used because it considers the possible 

virtual ankle dorsiflexion that was previously reported.1, 11 All 

results are described as mean values over six motion 

cycles.  

 

                                      

 

Figure 2: Schematic illustration of: (A) Prosthetic alignment. (B) Experimental setup with knee joint in full extension, (i) hip flexion angle 

(individual tube inclination was considered), (ii) distance between knee (extended) and big toe marker. (C) Experimental setup when knee 

joint is flexed, (iii) distance between knee (flexed) and big toe marker, (iv) minimum ground clearance. 

 

   

Figure 3: (A) Transfemoral amputee during level ground walking (example- the patient has given written informed consent for use of the 

picture). (B) Typical mean sagittal hip angle of transfemoral amputees while walking on level ground (n=6).17 Red area indicates assumed 

range where minimum ground clearance occurs. 

 

-35

-30

-25

-20

-15

-10

-5

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70 80 90 100

s
a
g
it
ta

l 
h
ip

 a
n
g
le

 [
 ]

t [%gait cycle]

BA B 

Hip-Axis 

900 mm 

 

 

 
 

Knee-Axis 

520 mm 

 

 

 

 
 

Heel 

10 mm 

 

Shank Shortening 

(ii)-(iv) 

 

 

Hip-Axis ap 

+10 mm 

https://doi.org/10.33137/cpoj.v3i1.33768


 

5 

Köhler T.M, Bellmann M, Blumentritt S. Polycentric Exoprosthetic Knee Joints – Extent of Shortening during Swing Phase. Canadian Prosthetics & Orthotics 
Journal. 2020;Volume3, Issue1, No.5. https://doi.org/10.33137/cpoj.v3i1.33768 

ISSN: 2561-987X SWING SHORTENING IN POLYCENTRIC KNEE JOINTS 

Köhler et al. 2020 

 
CPOJ 

 
Validation of the measurement method 

Due to minimal changes in the prosthetic alignment, setup 

alignment and marker placement, deviations in the 

investigated parameters might occur. To validate the 

reproducibility of the measurement method, the previously 

described procedure (1. prosthetic alignment, 2. 

experimental setup and application of reflective marker, 3. 

experimental procedure) was repeated 3 times (including 15 

motion cycles each) for one representative hip flexion 

position (25°) involving the 3R60 and C-Leg.  

• Evaluation of the inter-test reliability: 

For each repetition, the mean was calculated for lower leg 

shortening and minimum ground clearance. An ANOVA 

based reliability measure was calculated with a tolerance 

threshold of SD=1.5 mm, given the following formula:  

o Variance explained by threshold, assuming an 

equal distribution of means 𝑥�̅� over the range of 

their average �̅� ±1.5 mm: 𝑉𝑎𝑟𝑡ℎ𝑟𝑒𝑠ℎ𝑜𝑙𝑑 =  
1

12
(1.5 −

(−1.5))2 = 0.75 

o Reliability = 𝑚𝑖𝑛 [1;  1 − 
𝑉𝑎𝑟𝑏𝑒𝑡𝑤𝑒𝑒𝑛

𝑉𝑎𝑟𝑡𝑜𝑡𝑎𝑙
+

𝑉𝑎𝑟𝑡ℎ𝑟𝑒𝑠ℎ𝑜𝑙𝑑

𝑉𝑎𝑟𝑡𝑜𝑡𝑎𝑙
] 

with 𝑉𝑎𝑟𝑡𝑜𝑡𝑎𝑙 the total variance and 𝑉𝑎𝑟𝑏𝑒𝑡𝑤𝑒𝑒𝑛 the 

variance of the means.  

 

Subsequent ranges of the 3 mean values were calculated 

respectively:  

o minimum ground clearance: 1.6 mm (3R60) and 

0.6 mm (C-Leg) 
 

o lower leg shortening: 2.2 mm (3R60) and 0.3 mm 

(C-Leg) 

With a maximum range of 2.2 mm measured, the authors 

evaluate the measurement reliability as adequate for the 

statements made in this study (reliability coefficients 0.85-

1). 

• Evaluation of the intra-test reliability: 

Given the data from 15 motion cycles, the range and 

precision (half-length of the confidence interval for the 

mean) was calculated for each repetition, assuming that the 

point estimations of mean and variance were based on only 

6 measurements. This assumption allowed an estimation of 

the expected precision when using only 6 motion cycles 

instead of 15. The resulting maximum range (r) and 

minimum precision (pr) were: 

o minimum ground clearance: r = 0.4 mm, pr = 0.13 

mm (3R60);  r = 0.5 mm, pr = 0.13 mm (C-Leg) 
 

o lower leg shortening: r = 0.6 mm, pr = 0.15 

mm(3R60); r = 0.4 mm, pr = 0.13 mm (C-leg) 

The evaluation of the intra-test reliability showed that the 

rigid stationary device offers a constant motion of the 

prosthesis (deviation for measured parameters 

approximately SD=0.3 mm). Thus, mean values over six 

motion cycles seem to provide sufficient precision (0.06-

0.15 mm). 

RESULTS 

Lower leg shortening 

Except for the Total Knee and Allux, each polycentric knee 

joint generated a shortening of the lower leg segment during 

minimal ground clearance for all hip angles investigated. 

Shortening of up to 14.7 (SD=0.0) mm was measured for 

the 3R46/3R55. The Total Knee elongated over all 

investigated hip angles with a maximum of 4.4 (SD=0.0)  

mm at 30° hip flexion. The Allux knee joint elongated (max. 

0.5 mm) at a hip flexion angle of 25° and 30°. The 

shortening tends to decrease with increasing hip flexion 

angles for all joints expect for the KX06 and 3R46/3R55 

(Figure 4).  

Minimum ground clearance 

When comparing the results of ground clearances, the 

minimum value measured at each hip flexion angle was set 

to 0 mm. The lowest ground clearance was identified with 

the JT22 for all hip flexion angles. In comparison, the 3R60 

provides up to 25.4 (SD=0.0) mm greater ground clearance. 

At 15° hip flexion, 6 polycentric knee joints showed lower 

ground clearance than the C-Leg (12 mm with FP 1 / Knee 

+5). At 25° and 30° hip flexion, 8 polycentric knee joints 

demonstrated lower ground clearance than this 

monocentric knee joint (Figure 4). 

Comparison of different alignment methods 

A comparison of different alignment methods with regards 

to toe clearance was conducted for the Allux and C-Leg. At 

15° hip flexion, the Allux showed 4.1 mm greater ground 

clearance with the FP 1 than with the FP 2 and 4.3 mm 

greater ground clearance at 30° hip flexion, respectively. 

With the C-Leg, ground clearance at 15° hip flexion was 6.6 

mm greater with the FP 1 / Knee +5 than with the FP 2 / 

Knee 0 and 8.2 mm greater at 30° hip flexion, respectively 

(Figure 4). 

DISCUSSION  

The aim of this study was to investigate whether polycentric 

knee joints provide a substantial shortening of the lower leg 

segment during swing phase. The results indicate that this 

is not valid for each polycentric design. Due to the individual 

length and orientation of the linkages of each polycentric 

knee joint investigated a large variation of shortening effects 

were observed. As mentioned by Anand et al.18, a higher 

ICR seems to result in higher ground clearance.19 This 

should be considered in the development of the geometric 

design of a polycentric knee joint.18 However, this 

correlation could not be observed. Polycentric knee joints 

https://doi.org/10.33137/cpoj.v3i1.33768


 

6 

Köhler T.M, Bellmann M, Blumentritt S. Polycentric Exoprosthetic Knee Joints – Extent of Shortening during Swing Phase. Canadian Prosthetics & Orthotics 
Journal. 2020;Volume3, Issue1, No.5. https://doi.org/10.33137/cpoj.v3i1.33768 

ISSN: 2561-987X SWING SHORTENING IN POLYCENTRIC KNEE JOINTS 

Köhler et al. 2020 

 
CPOJ 

 
with a more proximally located ICR (Figure 1: Total Knee, 

JT22, 3R106) demonstrate lower leg shortening of up to 6.0 

(SD=0.0 mm) or even elongation of up to 4.4 (SD=0.0) mm. 

The 3 polycentric knee joints demonstrating the greatest 

shortening of the lower leg segment (Figure 1: KX06, 3R60, 

3R46/3R55) share other similarities (Figure 3, Table 2):  The 

posterior linkage is relatively long and tilted anteriorly. The 

ICR is (compared to the Total Knee, JT22, 3R106) more 

distally and more anteriorly located (close to the longitudinal 

axis of the knee joint). These knee joints reached 7.0 

(SD=0.0) mm up to 14.7 (SD=0.0) mm lower leg shortening. 

Furthermore, the impact of the prosthetic alignment on 

ground clearance was considerable. Thus, some of the 

knee joints generated relatively minor ground clearance 

even though they shortened to a large extent of up to 14.7 

(SD=0.0) mm.   

 

 

    

 

 

 

-6 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26

Total Knee

C-Leg (FP 1 / Knee 0)

C-Leg (FP 1 / Knee +5)

3R45/3R95

C-Leg (FP 2 / Knee 0)

TGK-4P01P

Allux (FP 1)

Allux (FP 2)

TK-4P00S

3R106

OH5/KH5

JT22

KX06

OHP3/KHP3

3R60

3R46/3R55

shortening/ground clearance [mm]

A

-6 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26

Total Knee

C-Leg (FP 1 / Knee 0)

C-Leg (FP 1 / Knee +5)

3R45/3R95

C-Leg (FP 2 / Knee 0)

TGK-4P01P

Allux (FP 1)

Allux (FP 2)

TK-4P00S

3R106

OH5/KH5

JT22

KX06

OHP3/KHP3

3R60

3R46/3R55

shortening/ground clearance [mm]

B

-6 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26

Total Knee

C-Leg (FP 1 / Knee 0)

C-Leg (FP 1 / Knee +5)

3R45/3R95

C-Leg (FP 2 / Knee 0)

TGK-4P01P

Allux (FP 1)

Allux (FP 2)

TK-4P00S

3R106

OH5/KH5

JT22

KX06

OHP3/KHP3

3R60

3R46/3R55

shortening/ground clearance [mm]

C

-6 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26

Total Knee

C-Leg (FP 1 / Knee 0)

C-Leg (FP 1 / Knee +5)

3R45/3R95

C-Leg (FP 2 / Knee 0)

TGK-4P01P

Allux (FP 1)

Allux (FP 2)

TK-4P00S

3R106

OH5/KH5

JT22

KX06

OHP3/KHP3

3R60

3R46/3R55

shortening/ground clearance [mm]

D

Figure 4: Shortening of the lower leg segment (black) and ground clearance (grey) at the instance of minimal ground clearance (A) 15°, 

(B) 20°, (C) 25° and (D) 30° hip flexion; mean with standard deviation. 

 

     Shortening of the lower leg segment                                    Ground clearance 

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7 

Köhler T.M, Bellmann M, Blumentritt S. Polycentric Exoprosthetic Knee Joints – Extent of Shortening during Swing Phase. Canadian Prosthetics & Orthotics 
Journal. 2020;Volume3, Issue1, No.5. https://doi.org/10.33137/cpoj.v3i1.33768 

ISSN: 2561-987X SWING SHORTENING IN POLYCENTRIC KNEE JOINTS 

Köhler et al. 2020 

 
CPOJ 

 

The a-p position of the knee joint caused by the alignment 

affects the ground clearance in two ways: The more 

posterior the knee joint, the 

1. closer the knee center of rotation is to the ground 

during hip flexion. This effect increases with increasing 

hip flexion angles due to the more distally located 

trajectory of the knee reference point (Figure 5 A-B). 

2. greater the distance between the knee center of 

rotation and the big toe and this results in a longer 

prosthesis at the instance of minimal ground clearance 

(Figure 5 C-D). 

This substantive alignment effect can be clearly seen with 

the 3R95 monocentric knee joint which is aligned 20 mm 

more posteriorly than the monocentric C-Leg (FP 1 / Knee 

+5) and this results in 16.5 mm less ground clearance at 30° 

hip flexion. 

Thus, ground clearance during swing phase is enhanced 

when the knee joint is placed in a more anterior position. As 

an example, one monocentric knee joint (C-Leg, FP 1 / 

Knee +5) generated greater ground clearance then up to 8 

polycentric knee joints due to its specific alignment 

depending on hip flexion angle.  

Table 2: Evaluation of the geometric design of the polycentric knee joints investigated based on Figure 1 (visual comparison of proportions).  

 

Knee joint ICR height*  
ICR A-P 
position  

Length anterior 
linkage 

Length posterior 
linkage 

Tilt anterior 
linkage 

Tilt posterior 
linkage 

LLS (25°) 
[mm] 

Total Knee high posterior short short backward  backward  -3.9 (SD=0.1) 

Allux (FP 2) low centered short short backward  forward  0.0 (SD=0.0) 

TGK-4P01P low posterior short short backward backward 0.3 (SD=0.1) 

3R106 high posterior long short backward backward 2.8 (SD=0.0) 

TK-4P00S medium posterior short short backward backward 3.7 (SD=0.0) 

OH5/KH5 
below knee 
joint 

anterior long short backward backward 4.6 (SD=0.1) 

JT22 high posterior medium short backward backward 5.2 (SD=0.2) 

OHP3/KHP3 
below knee 
joint 

anterior long short backward backward 7.4 (SD=0.2) 

KX06 low centered long medium backward forward 7.4 (SD=0.0) 

3R60 medium centered long medium backward forward 10.8 (SD=0.1) 

3R46/3R55 medium centered long long backward forward 14.2 (SD=0.1) 

 

*high-low: ICR above knee joint; high: more proximally located, low: more distally located. Abbreviations: ICR instantaneous center of 

rotation, A-P anterior posterior, LLS lower leg shortening 

 

 

Figure 5: Effect of a-p positioning of the knee and foot components (A) experimental set up, (B) schematic diagram of the effect of a-p 

positioning of the knee, (i) hip flexion angle, (ii) gain of ground clearance, (iii) loss of ground clearance, (C) prosthetic alignment, (D) schematic 

diagram of shortening and elongation of the shank, respectively, (iv) depending on the position of the knee and foot. 

 

Knee  

ap - position 

 

Foot  

ap - position 

https://doi.org/10.33137/cpoj.v3i1.33768


 

8 

Köhler T.M, Bellmann M, Blumentritt S. Polycentric Exoprosthetic Knee Joints – Extent of Shortening during Swing Phase. Canadian Prosthetics & Orthotics 
Journal. 2020;Volume3, Issue1, No.5. https://doi.org/10.33137/cpoj.v3i1.33768 

ISSN: 2561-987X SWING SHORTENING IN POLYCENTRIC KNEE JOINTS 

Köhler et al. 2020 

 
CPOJ 

 
Additionally, the effect of the foot position was investigated 

and resulted in the following observation: 

The more anterior the position of the foot, the longer the 

forefoot and the longer the distance between the knee 

center of rotation and the big toe. Hence, ground clearance 

decreases during swing with a more anteriorly positioned 

foot (Figure 5 C-D). As a conclusion, the FP 1 exhibited a 

considerable advantage over the FP 2 for the two knee 

joints that were investigated with these two alignment 

methods (C-Leg, Allux). 

In this study the extent of geometric shortening of the lower 

leg segment with different polycentric knee joints, as well as 

the impact of prosthetic alignment on the resulting ground 

clearance was investigated. Another potential aspect 

regarding the resulting ground clearance, not being 

investigated in this study, is the property of the swing phase 

control of the prosthetic knee joint. Based on the 

investigation of Winter, small differences in knee flexion 

angle considerably affect ground clearance. 1.4° difference 

in the knee flexion angle resulted in a difference of 4.5 mm 

ground clearance, seen in non-amputees.15 Depending on 

the individual swing phase control (appropriate adjustment 

of the swing flexion and extension resistance) of the 

prosthetic knee joint, the knee angle progression during 

swing phase can vary significantly.20,21 Thus, the swing 

phase control might have an influence on the resulting 

ground clearance. Therefore, further experiments with the  

knee joints presented in this study are suggested to clarify 

this additional aspect. 

Limitations 

The prosthetic and experimental setup as well as the marker 

positioning was adjusted with the greatest care. However, 

minimal deviations might occur, as seen in the validation of 

the measurement method. Due to these effects, non-

realistic shortenings of the shank were measured even for 

monocentric knee joints (e.g. C-Leg 4, (SD=0.4) mm). The 

authors assume an overall accuracy of SD=1.1 mm for the 

investigated parameters. This assumption is based on the 

validation of the entire measurement method. Nevertheless, 

this accuracy is sufficient for the statements that were made 

in this study. 

CONCLUSION 

In summary, not all polycentric knee joints shorten 

appreciably at the instant when a stumble might occur. 

Thus, the previously stated functional advantage of greater 

ground clearance for patients must be reconsidered. A 

slightly more anterior position of the knee joint or a more 

posterior position of the foot can compensate for or even 

exceed the extent of the geometric shortening of the shank 

of some polycentric knee joints. 

 

ACKNOWLEDGEMENTS 

The authors thank Prof. Dr. Dieter Rosenbaum, Greg 

Schneider, Lena Uhlenberg and Aideen Curran for their 

valuable contribution to the preparation of this article. 

DECLARATION OF CONFLICTING 

INTERESTS 

Mr. Thomas Maximilian Köhler and Dr. Malte Bellmann are 

employees of Ottobock SE & Co. KGaA. 

AUTHOR CONTRIBUTION 

Each of the authors concurs with the content in the final 

manuscript. 
  

• Mr. Thomas Maximilian Köhler, MSc: study design, 

execution of the gait lab measurements, data analysis 

and interpretation, drafting of the article 

ORCID: https://orcid.org/0000-0002-5063-121X 
 

• Dr. Malte Bellmann: study design, execution of the gait 

lab measurements, data analysis and interpretation, 

drafting of the article. 

ORCID: https://orcid.org/0000-0002-5002-9245 
 

• Prof. Dr. Siegmar Blumentritt: drafting of the article. 

ORCID: https://orcid.org/0000-0003-4190-6098 

 

SOURCES OF SUPPORT 

This research received no specific grant from any agency in 

the public, commercial or not-for-profit sectors. 

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Köhler T.M, Bellmann M, Blumentritt S. Polycentric Exoprosthetic Knee Joints – Extent of Shortening during Swing Phase. Canadian Prosthetics & Orthotics 
Journal. 2020;Volume3, Issue1, No.5. https://doi.org/10.33137/cpoj.v3i1.33768 

ISSN: 2561-987X SWING SHORTENING IN POLYCENTRIC KNEE JOINTS 

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https://doi.org/10.33137/cpoj.v3i1.33768
https://www.worldcat.org/title/above-knee-prosthesis-technology/oclc/48194801
https://www.worldcat.org/title/above-knee-prosthesis-technology/oclc/48194801
https://est-kl.com/images/PDF/Vicon/Bonita_Optical.pdf
https://www.iso.org/standard/38708.html

