In current literature, databases of healthy knee anatomy and function are confined to small sample sizes or singular imaging modalities. In this study, the investigator propose constructing a unique and large-scale database of healthy Caucasian knee anatomy and function by utilizing a combination of ultra-high resolution capabilities from photon-counting CT, lower limb weightbearing orthogonal imaging (EOS) of full leg, and non-invasive ultrasound-based functional testing of the knee. This study is a collaborative effort between the orthopaedics, radiology and biomechanical engineering departments in which the same prospectively acquired database will be used for multiple independent research questions.

Total knee arthroplasty (TKA) is the mainstay of treatment of end-stage osteoarthritis. Despite recent robotic advancements in TKA surgical technique, satisfaction rates remain low compared to the golden standard of hip arthroplasty surgery. Personalization in TKA, i.e. accurate recreation of individual anatomy and biomechanical properties of the knee joint, is increasingly recognized as an important variable in achieving satisfaction after TKA. Therefore, this study aims to gain foundational knowledge necessary for further research in anatomical and biomechanical personalization in TKA.

Anatomical personalization in TKA aims at recreating the pre-arthritic anatomy of each patient. The combination of native knee joint-line anatomy as well as native overall lower limb alignment anatomy can be referred to as the concept of 'constitutional anatomy'.

Historically, anatomical personalization in TKA focused primarily on variable tibiofemoral anatomy in the coronal plane. This led to the groundbreaking work, introducing the concept of coronal plane alignment of the knee (CPAK) phenotyping. This classification categorized knees into nine coronal morphologies based on limb alignment and knee joint line obliquity.

Another facet of personalization lies in biomechanical components, particularly the variable native mediolateral ligamentous balance of the knee (referred to as 'constitutional balance'). Presently, personalized ligamentous balancing in TKA remains largely unexplored, with surgeons aiming for a standardized balancing target regardless of patient-specific variations. This systematic approach towards knee balancing is a result of difficulty in predicting constitutional balance in arthritic knees, as progressive osteoarthritis alters the native ligamentous balance due to osteophyte formation, ligamentous wear and contractures. Given that joint biomechanics result (in part) from threedimensional joint line anatomy, it is plausible that variable constitutional balance might directly relate to variable constitutional anatomy. This assumption will be further evaluated in this study and, if confirmed, would imply the clinical possibility of determining constitutional balance by determining constitutional anatomical phenotype.

This study aims to identify phenotypes of constitutional anatomy and balance and correlate both by analysis of a prospectively acquired database of Caucasian volunteers with 'healthy knees' (see inclusion criteria for definition of a 'healthy knee').

For the anatomical analysis, a combination of photon-counting CT (PC-CT) and EOS full leg imaging will be used. The combination of these novel imaging modalities allows for a highly detailed evaluation of knee joint line anatomy combined with analysis of weight-bearing lower limb alignment in all three anatomic planes, whilst ensuring minimal exposure levels to ionizing radiation.

For the analysis of constitutional balance, this study will use an ultrasound-based technique. Ultrasound-based gap analysis is a non-invasive and previously validated method of determining medio-lateral laxity of the knee joint.

Inclusion Criteria:

• Aged 20-35 years old.

• 'Caucasian':

  1. Self-identification as Caucasian or of European descent.
  2. Self-identification as having 'fair' skin tone.
  3. Both parents and all grandparents are of Caucasian or European descent.

• Healthy knee and lower limb:

  1. No bony orthopaedic or trauma history in the lower limb (pelvis - foot).
  2. No ligamentary or soft tissue trauma history in the knee.
  3. No ligamentary or soft tissue trauma in the lower limb requiring cast immobilization.

Exclusion Criteria:

• Individuals identifying with a race or ethnicity other than Caucasian or European descent.
• Participants with known mixed racial or ethnic backgrounds that include non-Caucasian ancestry.
• Participant uncertainty of pregnancy status.
• Open wounds over the knee joint.